Change attach_inline_image default domain from hostname to "inline".

This avoids problems with ESPs that don't distinguish *Content-ID*
from attachment filename, where a local hostname ending in ".com" could
cause Gmail to block messages sent with inline attachments.
(Mailgun, Mailjet, Mandrill and SparkPost have APIs affected by this.)

Fixes #112.

tests/test_message.py

@@ -0,0 +1,31 @@
+from django.core.mail import EmailMultiAlternatives
+from django.test import SimpleTestCase
+from mock import patch
+
+from anymail.message import attach_inline_image
+
+from .utils import AnymailTestMixin, sample_image_content
+
+
+class InlineImageTests(AnymailTestMixin, SimpleTestCase):
+    def setUp(self):
+        self.message = EmailMultiAlternatives()
+        super(InlineImageTests, self).setUp()
+
+    @patch("email.utils.socket.getfqdn")
+    def test_default_domain(self, mock_getfqdn):
+        """The default Content-ID domain should *not* use local hostname"""
+        # (This avoids problems with ESPs that re-use Content-ID as attachment
+        # filename: if the local hostname ends in ".com", you can end up with
+        # an inline attachment filename that causes Gmail to reject the message.)
+        mock_getfqdn.return_value = "server.example.com"
+        cid = attach_inline_image(self.message, sample_image_content())
+        self.assertRegex(cid, r"[\w.]+@inline",
+                         "Content-ID should be a valid Message-ID, "
+                         "but _not_ @server.example.com")
+
+    def test_domain_override(self):
+        cid = attach_inline_image(self.message, sample_image_content(),
+                                  domain="example.org")
+        self.assertRegex(cid, r"[\w.]+@example\.org",
+                         "Content-ID should be a valid Message-ID @example.org")