Support for heterogeneous arrays

[wajda]

When serializing an array of an abstract class or interface, a type hint should be added to the items, just like it's done for generic lists.

[wajda]

@mpv1989
I've created a pull request that fixes this issue. Can you please take a look?

Also I would like to add support for generic arrays and improve API for custom serializers. I'll create separate issues for that.

[rashtao]

@wajda is this issue fixed by #13?

[wajda]

@rashtao Yes, please close it.