Mailgun, SparkPost: support multiple from_email addresses

[RFC-5322 allows](https://tools.ietf.org/html/rfc5322#section-3.6.2)
multiple addresses in the From header.

Django's SMTP backend supports this, as a single comma-separated
string (*not* a list of strings like the recipient params):

    from_email='one@example.com, two@example.com'
    to=['one@example.com', 'two@example.com']

Both Mailgun and SparkPost support multiple From addresses
(and Postmark accepts them, though truncates to the first one
on their end). For compatibility with Django -- and because
Anymail attempts to support all ESP features -- Anymail now
allows multiple From addresses, too, for ESPs that support it.

Note: as a practical matter, deliverability with multiple
From addresses is pretty bad. (Google outright rejects them.)

This change also reworks Anymail's internal ParsedEmail object,
and approach to parsing addresses, for better consistency with
Django's SMTP backend and improved error messaging.

In particular, Django (and now Anymail) allows multiple email
addresses in a single recipient string:

    to=['one@example.com', 'two@example.com, three@example.com']
    len(to) == 2  # but there will be three recipients

Fixes #60

tests/test_mailgun_backend.py

@@ -18,7 +18,9 @@ from django.test import SimpleTestCase
 from django.test.utils import override_settings
 from django.utils.timezone import get_fixed_timezone, override as override_current_timezone
 
-from anymail.exceptions import AnymailAPIError, AnymailRequestsAPIError, AnymailUnsupportedFeature
+from anymail.exceptions import (
+    AnymailAPIError, AnymailInvalidAddress,
+    AnymailRequestsAPIError, AnymailUnsupportedFeature)
 from anymail.message import attach_inline_image_file
 
 from .mock_requests_backend import RequestsBackendMockAPITestCase, SessionSharingTestCasesMixin
@@ -53,7 +55,7 @@ class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
         data = self.get_api_call_data()
         self.assertEqual(data['subject'], "Subject here")
         self.assertEqual(data['text'], "Here is the message.")
-        self.assertEqual(data['from'], "from@example.com")
+        self.assertEqual(data['from'], ["from@example.com"])
         self.assertEqual(data['to'], ["to@example.com"])
 
     def test_name_addr(self):
@@ -68,7 +70,7 @@ class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
             bcc=['Blind Copy <bcc1@example.com>', 'bcc2@example.com'])
         msg.send()
         data = self.get_api_call_data()
-        self.assertEqual(data['from'], "From Name <from@example.com>")
+        self.assertEqual(data['from'], ["From Name <from@example.com>"])
         self.assertEqual(data['to'], ['Recipient #1 <to1@example.com>', 'to2@example.com'])
         self.assertEqual(data['cc'], ['Carbon Copy <cc1@example.com>', 'cc2@example.com'])
         self.assertEqual(data['bcc'], ['Blind Copy <bcc1@example.com>', 'bcc2@example.com'])
@@ -86,7 +88,7 @@ class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
         data = self.get_api_call_data()
         self.assertEqual(data['subject'], "Subject")
         self.assertEqual(data['text'], "Body goes here")
-        self.assertEqual(data['from'], "from@example.com")
+        self.assertEqual(data['from'], ["from@example.com"])
         self.assertEqual(data['to'], ['to1@example.com', 'Also To <to2@example.com>'])
         self.assertEqual(data['bcc'], ['bcc1@example.com', 'Also BCC <bcc2@example.com>'])
         self.assertEqual(data['cc'], ['cc1@example.com', 'Also CC <cc2@example.com>'])
@@ -249,6 +251,20 @@ class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
         data = self.get_api_call_data()
         self.assertNotIn('to', data)
 
+    def test_multiple_from_emails(self):
+        """Mailgun supports multiple addresses in from_email"""
+        self.message.from_email = 'first@example.com, "From, also" <second@example.com>'
+        self.message.send()
+        data = self.get_api_call_data()
+        self.assertEqual(data['from'], ['first@example.com',
+                                        '"From, also" <second@example.com>'])
+
+        # Make sure the far-more-likely scenario of a single from_email
+        # with an unquoted display-name issues a reasonable error:
+        self.message.from_email = 'Unquoted, display-name <from@example.com>'
+        with self.assertRaises(AnymailInvalidAddress):
+            self.message.send()
+
     def test_api_failure(self):
         self.set_mock_response(status_code=400)
         with self.assertRaisesMessage(AnymailAPIError, "Mailgun API response 400"):
@@ -488,15 +504,20 @@ class MailgunBackendRecipientsRefusedTests(MailgunBackendMockAPITestCase):
         "message": "'to' parameter is not a valid address. please check documentation"
     }"""
 
+    # NOTE: As of Anymail 0.10, Anymail catches actually-invalid recipient emails
+    # before attempting to pass them along to the ESP, so the tests below use technically
+    # valid emails that would actually be accepted by Mailgun. (We're just making sure
+    # the backend would correctly handle the 400 response if something slipped through.)
+
     def test_invalid_email(self):
         self.set_mock_response(status_code=400, raw=self.INVALID_TO_RESPONSE)
-        msg = mail.EmailMessage('Subject', 'Body', 'from@example.com', to=['not a valid email'])
+        msg = mail.EmailMessage('Subject', 'Body', 'from@example.com', to=['not-really@invalid'])
         with self.assertRaises(AnymailAPIError):
             msg.send()
 
     def test_fail_silently(self):
         self.set_mock_response(status_code=400, raw=self.INVALID_TO_RESPONSE)
-        sent = mail.send_mail('Subject', 'Body', 'from@example.com', ['not a valid email'],
+        sent = mail.send_mail('Subject', 'Body', 'from@example.com', ['not-really@invalid'],
                               fail_silently=True)
         self.assertEqual(sent, 0)