Unfreeze event before serializing with ujson

In newer versions of https://github.com/esnme/ultrajson, ujson does not
serialize frozendicts (introduced in esnme/ultrajson@53f85b1). Although
the PyPI version is still 1.35, Fedora ships with a build from commit
esnme/ultrajson@2f1d487. This causes the serialization to fail if the
distribution-provided package is used.

This runs the event through the unfreeze utility before serializing it.

Thanks to @ignatenkobrain for tracking down the root cause.

fixes #2351

Signed-off-by: Jeremy Cline <jeremy@jcline.org>

synapse/handlers/message.py

@@ -26,6 +26,7 @@ from synapse.types import (
 from synapse.util.async import run_on_reactor, ReadWriteLock, Limiter
 from synapse.util.logcontext import preserve_fn
 from synapse.util.metrics import measure_func
+from synapse.util.frozenutils import unfreeze
 from synapse.visibility import filter_events_for_client
 
 from ._base import BaseHandler
@@ -503,7 +504,7 @@ class MessageHandler(BaseHandler):
 
         # Ensure that we can round trip before trying to persist in db
         try:
-            dump = ujson.dumps(event.content)
+            dump = ujson.dumps(unfreeze(event.content))
             ujson.loads(dump)
         except:
             logger.exception("Failed to encode content: %r", event.content)