Support Properties subclasses in GsonTypes.getMapKeyAndValueTypes

[panic08]

There is a TODO in this method that calls to test it with the Properties subclasses. They are supposed to return new Type[] {String.class, String.class}, just like if you cast the Properties class itself. But still when you pass a subclass of Properties, such as CustomProperties, the method returns new Type[] {Object.class, Object.class}, which is incorrect. Here I tried to fix this problem by using isAssignableFrom() and also wrote tests which indicate that the problem is solved

[eamonnmcmanus]

Thanks! This does seem like the sort of thing where existing code might conceivably be depending on the current behaviour. I'll run this against all of Google's internal tests and if nothing fails we can go ahead with the change.

[eamonnmcmanus]

I ran this against all of Google's internal tests and there were no failures, so I think it is pretty safe to merge.