Fix bug where we didn't always get 'prev_content' key

synapse/api/events/__init__.py

@@ -57,6 +57,7 @@ class SynapseEvent(JsonEncodedObject):
         "state_key",
         "required_power_level",
         "age_ts",
+        "prev_content",
     ]
 
     internal_keys = [
@@ -172,10 +173,6 @@ class SynapseEvent(JsonEncodedObject):
 
 class SynapseStateEvent(SynapseEvent):
 
-    valid_keys = SynapseEvent.valid_keys + [
-        "prev_content",
-    ]
-
     def __init__(self, **kwargs):
         if "state_key" not in kwargs:
             kwargs["state_key"] = ""