Fix SYJS-38 - Events stuck in 'sending' state

v1 used to clobber events after sending so `.status` would be `null`. v2 is
smarter and just clobbers the `.event` data so references to the local echo
event would reflect the new event. However, the `.status` in this case would
still have the old value (SENDING), so make sure to reset it after the 200 OK
from sending the event.

lib/client.js

@@ -1073,6 +1073,7 @@ function _sendEvent(client, room, event, callback) {
                     matchingEvent.event.type = event.event.type;
                 }
                 room.removeEvents([localEventId]);
+                matchingEvent.status = null; // make sure it's still marked as sent
             }
             else {
                 room.removeEvents([localEventId]);