NetworkMenu: fix parameters to _findConnection

NMDevice._findConnection expects an uuid as parameter, but
checkConnection was passing a NMConnection object. This caused
exists to be always false, thus the connection was added again every
time the 'updated' signal was emitted.

https://bugzilla.gnome.org/show_bug.cgi?id=645702

js/ui/status/network.js

@@ -424,7 +424,7 @@ NMDevice.prototype = {
     },
 
     checkConnection: function(connection) {
-        let exists = this._findConnection(connection) != -1;
+        let exists = this._findConnection(connection._uuid) != -1;
         let valid = this.connectionValid(connection);
         if (exists && !valid)
             this.removeConnection(connection);