SendGrid: fix inbound webhook Unicode error when not utf-8

Fix a crash or text-mangling issue when an inbound message
uses a charset other than utf-8 for its text or html body,
and SendGrid's "post raw" inbound parse option is *not*
enabled.

Update docs to recommend "post raw" option.

Fixes #187

tests/test_sendgrid_inbound.py

@@ -1,3 +1,5 @@
+# -*- coding: utf-8 -*-
+
 import json
 from textwrap import dedent
 
@@ -9,7 +11,7 @@ from anymail.inbound import AnymailInboundMessage
 from anymail.signals import AnymailInboundEvent
 from anymail.webhooks.sendgrid import SendGridInboundWebhookView
 
-from .utils import sample_image_content, sample_email_content
+from .utils import dedent_bytes, sample_image_content, sample_email_content
 from .webhook_cases import WebhookTestCase
 
 
@@ -183,3 +185,59 @@ class SendgridInboundTestCase(WebhookTestCase):
         self.assertEqual(message.subject, 'Raw MIME test')
         self.assertEqual(message.text, u"It's a body\N{HORIZONTAL ELLIPSIS}\n")
         self.assertEqual(message.html, u"""<div dir="ltr">It's a body\N{HORIZONTAL ELLIPSIS}</div>\n""")
+
+    def test_inbound_charsets(self):
+        # Captured (sanitized) from actual SendGrid inbound webhook payload 7/2020,
+        # using a test message constructed with a variety of charsets:
+        raw_post = dedent_bytes(b"""\
+            --xYzZY
+            Content-Disposition: form-data; name="headers"
+
+            Date: Fri, 24 Jul 2020 16:43:46 UTC
+            To: =?utf-8?q?R=C3=A9cipiendaire_pr=C3=A9cieux?= <inbound@sg.example.com>
+            From: =?utf-8?q?Op=C3=A9rateur?= de test <sender@example.com>
+            Subject: =?cp850?q?Como_usted_pidi=A2?=
+
+            --xYzZY
+            Content-Disposition: form-data; name="subject"
+
+            Como usted pidi\xa2
+            --xYzZY
+            Content-Disposition: form-data; name="to"
+
+            R\xc3\xa9cipiendaire pr\xc3\xa9cieux <inbound@sg.example.com>
+            --xYzZY
+            Content-Disposition: form-data; name="html"
+
+            <p>\xbfEsto se ve como esperabas?</p>
+            --xYzZY
+            Content-Disposition: form-data; name="from"
+
+            Op\xc3\xa9rateur de test <sender@example.com>
+            --xYzZY
+            Content-Disposition: form-data; name="text"
+
+            Test the ESP\x92s inbound charset handling\x85
+            --xYzZY
+            Content-Disposition: form-data; name="charsets"
+
+            {"to":"UTF-8","cc":"UTF-8","html":"iso-8859-1","subject":"cp850","from":"UTF-8","text":"windows-1252"}
+            --xYzZY--
+            """).replace(b"\n", b"\r\n")
+
+        response = self.client.post('/anymail/sendgrid/inbound/', data=raw_post,
+                                    content_type="multipart/form-data; boundary=xYzZY")
+        self.assertEqual(response.status_code, 200)
+        kwargs = self.assert_handler_called_once_with(self.inbound_handler, sender=SendGridInboundWebhookView,
+                                                      event=ANY, esp_name='SendGrid')
+        event = kwargs['event']
+        message = event.message
+
+        self.assertEqual(message.from_email.display_name, u"Opérateur de test")
+        self.assertEqual(message.from_email.addr_spec, "sender@example.com")
+        self.assertEqual(len(message.to), 1)
+        self.assertEqual(message.to[0].display_name, u"Récipiendaire précieux")
+        self.assertEqual(message.to[0].addr_spec, "inbound@sg.example.com")
+        self.assertEqual(message.subject, u"Como usted pidió")
+        self.assertEqual(message.text, u"Test the ESP’s inbound charset handling…")
+        self.assertEqual(message.html, u"<p>¿Esto se ve como esperabas?</p>")