telepathyClient: Acknowledge messages before closing

While a channel has pending messages, it will pop up again when
dismissed. That is clearly not what users expect, so clear them
out first before closing a channel.

https://bugzilla.gnome.org/show_bug.cgi?id=747991

js/ui/components/telepathyClient.js

@@ -469,6 +469,7 @@ const ChatSource = new Lang.Class({
 
     destroy: function(reason) {
         if (this._client.is_handling_channel(this._channel)) {
+            this._ackMessages();
             // The chat box has been destroyed so it can't
             // handle the channel any more.
             this._channel.close_async(function(channel, result) {