Adds support for ImmutableMap in jclouds gson

Specifically making sure deserialization works properly.
Waiddaminute...if I'm understanding this correctly, `MapTypeAdapter` _already_ [builds an immutable map](https://github.com/jclouds/jclouds/blob/master/core/src/main/java/org/jclouds/json/internal/NullFilteringTypeAdapterFactories.java#L299-309)? If that is correct, what is the difference here?

And if we **do** need a different adapter, is there any way to cut down on the duplication between IMTA/MTA and IMTAF/MTAF?