Convert the id value to a String before comparison

When trying to use the plugin on ios to display a simple local notification, i.e.

```
window.plugin.notification.local.add({
	id: "1",
	message: "test",
	title: "Hello"
});
```
The application throws the following exception

```
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFNumber isEqualToString:]: unrecognized selector sent to instance 0x17d7b4b0'
```
It appears that ios is returning the id as an Integer, so the quick and dirty fix would be to cast to a String before comparing.