Clean up a codepath that was only used for crypto messages

Transmission of encrypted messages was happening somewhat differently to
normal messages. In particular, we weren't copying the 'unsigned' field when we
got the remote-echo, which meant the 'sync' code didn't correctly match up the
echo with the original.

The separate codepath was becoming a thorn in my side, so fix things up to
bring it back in line.

lib/models/room.js

@@ -621,8 +621,13 @@ Room.prototype._addLiveEvents = function(events) {
                     this._eventIdToTimeline[events[i].getId()] = existingTimeline;
                 }
 
-                // replace the event source
+                // replace the event source, but preserve the original content
+                // and type in case it was encrypted (we won't be able to
+                // decrypt it, even though we sent it.)
+                var existingSource = existingEvent.event;
                 existingEvent.event = events[i].event;
+                existingEvent.event.content = existingSource.content;
+                existingEvent.event.type = existingSource.type;
 
                 // successfully sent.
                 existingEvent.status = null;