Update WindowsPhoneNotification.cs

! Fixed the & and end of navigationline bug. As the "paramValue" wasn't overwritten, the value was never replaced

PushSharp.WindowsPhone/WindowsPhoneNotification.cs

@@ -143,7 +143,7 @@ public override string PayloadToString()
 					var paramValue = sb.ToString();
 
 					if (!string.IsNullOrEmpty(paramValue) && paramValue.EndsWith("&"))
-						paramValue.Substring(0, paramValue.Length - 1);
+						paramValue = paramValue.Substring(0, paramValue.Length - 1);
 
 					if (!string.IsNullOrEmpty(paramValue))
 						toast.Add(new XElement(wp + "Param", paramValue));