Fixup calls to comput_event_signature

We currently pass FrozenEvent instead of `dict` to
`compute_event_signature`, which works by accident due to `dict(event)`
producing the correct result.

This fixes PR #4493 commit 855a151

synapse/federation/federation_server.py

@@ -322,7 +322,7 @@ def _on_context_state_request_compute(self, room_id, event_id):
             if self.hs.is_mine_id(event.event_id):
                 event.signatures.update(
                     compute_event_signature(
-                        event,
+                        event.get_pdu_json(),
                         self.hs.hostname,
                         self.hs.config.signing_key[0]
                     )

synapse/handlers/federation.py

@@ -1300,7 +1300,7 @@ def on_invite_request(self, origin, pdu):
 
         event.signatures.update(
             compute_event_signature(
-                event,
+                event.get_pdu_json(),
                 self.hs.hostname,
                 self.hs.config.signing_key[0]
             )