ActivityParser doesn't parse the JSON of incoming messages.

This is because when we validate the message, a little further
down the line, we need access to the exact content of the message.
Parsing it here would destroy the exact content, because Django
doesn't let you read request bodies twice.

kepi/bowler_pub/parsers.py

@@ -5,16 +5,29 @@
 # Licensed under the GNU Public License v2.
 
 """
-django-rest-framework provides a JSON parser, which
-does what we need for parsing ActivityPub. However,
-it baulks at the ActivityPub MIME type. So we have a
-subclass here with only the accepted MIME type changed.
+A custom django-rest-framework parser for ActivityPub.
 """
 
-from rest_framework.parsers import JSONParser
+import codecs
+from django.conf import settings
+from rest_framework.parsers import BaseParser
 
-class ActivityParser(JSONParser):
+class ActivityParser(BaseParser):
     """
     django-rest-framework parser for application/activity+json.
     """
     media_type = "application/activity+json"
+
+    def parse(self, stream, media_type=None, parser_context=None):
+
+        """
+        Traditionally, we'd be parsing the JSON here. But despite
+        the name, we don't parse the incoming stream here at all.
+        This is because validate() needs to know the exact content
+        in order to test the signature. So we just pass it back
+        as a bytestring.
+        """
+
+        result = stream.read()
+
+        return result