NetworkMenu: fix updating the access point strength

A notify signal does not include the new value of the property in
its signature, so the handler was trying to compare a GParamSpec with
a number when updating. Fix it to always retrieve the value from the
object.

https://bugzilla.gnome.org/show_bug.cgi?id=646443

js/ui/status/network.js

@@ -132,8 +132,8 @@ NMNetworkMenuItem.prototype = {
         }
     },
 
-    _updated: function(ap, strength) {
-        if (strength > this.bestAP.strength)
+    _updated: function(ap) {
+        if (ap.strength > this.bestAP.strength)
             this.bestAP = ap;
 
         this._signalIcon.icon_name = this._getIcon();