Webhooks: fix 403 Forbidden errors (csrf check)

* csrf_exempt must be applied to View.dispatch,
  not View.post.

* In base WebhookTestCase, enable Django test Client
  enforce_csrf_checks. (Test Client by default disables
  CSRF protection.)

Closes #19

tests/webhook_cases.py

@@ -6,7 +6,7 @@ from mock import create_autospec, ANY
 from anymail.exceptions import AnymailInsecureWebhookWarning
 from anymail.signals import tracking, inbound
 
-from .utils import AnymailTestMixin
+from .utils import AnymailTestMixin, ClientWithCsrfChecks
 
 
 def event_handler(sender, event, esp_name, **kwargs):
@@ -22,6 +22,8 @@ class WebhookTestCase(AnymailTestMixin, SimpleTestCase):
     - sets up basic auth by default (since most ESP webhooks warn if it's not enabled)
     """
 
+    client_class = ClientWithCsrfChecks
+
     def setUp(self):
         super(WebhookTestCase, self).setUp()
         # Use correct basic auth by default (individual tests can override):