glib.ProxyInterface async method calls do not properly return single results

[mpnordland]

Hello again!

When calling methods that return a single result, the asynchronous glib.ProxyInterface functions return a list with a single item.
The synchronous versions of those methods correctly return just the single result.

I believe the code from here in the synchronous call should be moved up to here in the asynchronous call.

I will have a pull request in shortly.

Thanks,
Micah