Fix for RuntimeTypeAdapterFactory.

[leangl]

RuntimeTypeAdapterFactory stopped working since the 2.0 release. This should fix it, I can provide the corresponding test cases if needed.

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[JakeWharton]

Test cases are required, yes.

[swankjesse]

This is working as designed. If you have a problem with the design, we can address that.

Basically RuntimeTypeAdapterFactory only triggers when the supertype is declared:

   Shape shape;

It doesn't trigger when a subtype is declared:

   Square square;

There’s not much reason to use RuntimeTypeAdapterFactory in that case.

[todoooo]

This pull request makes sense. The adapter factory interrogates the class type of the instance during serialisation, not the declared class type. The original implementation only works for deserialisation.

[leangl]

I signed it!

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[Marcono1234]

This was resolved by #2160. Thank you nonetheless for your pull request!