Handle "join" events properly

This handles the fact that when using the /sync endpoint, events for already
joined rooms are also in the "join" dictionary in the response. The firing of
the "joined" event is moved to client:_make_room() and done only the first
time that the room creation is requested.

This also fixes bogus behavior: previosly, when a message was sent to a room
there would be multiople places where the "joined" event would be fired, and
message events pushed multile times onto the room object, which would call
the event handlers more than once for each message. Ugh.

matrix/client.lua

@@ -369,15 +369,16 @@ function Client:join_room(room)
    else
       error("argument #1 must be a string or a room object")
    end
-   room = self:_make_room(response.room_id)
-   self:fire("joined", room)
-   return room
+   return self:_make_room(response.room_id)
 end
 
 function Client:_make_room(room_id)
-   assert(not self.rooms[room_id], "Room already exists")
-   local room = Room(self, room_id)
-   self.rooms[room_id] = room
+   local room = self.rooms[room_id]
+   if not room then
+      room = Room(self, room_id)
+      self.rooms[room_id] = room
+      self:fire("joined", room)
+   end
    return room
 end
 
@@ -440,7 +441,6 @@ end
 function Client:_sync_handle_room__join(room_id, data)
    local room = self:_make_room(room_id)
    room:_push_events(data.timeline.events)
-   self:fire("joined", room)
 end
 
 function Client:_sync_handle_room__invite(room_id, data)