Work around bugs in some servers

Closes #118

Author: tulir

mautrix/appservice/as_handler.py

@@ -237,6 +237,11 @@ async def _http_handle_transaction(self, request: web.Request) -> web.Response:
 
     @staticmethod
     def _fix_prev_content(raw_event: JSON) -> None:
+        try:
+            if raw_event["unsigned"] is None:
+                del raw_event["unsigned"]
+        except KeyError:
+            pass
         try:
             raw_event["unsigned"]["prev_content"]
         except KeyError: