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Add support for generic types in GsonHttpMessageConverter

This commit adds support to read and write generic types using the
Google Gson library. GsonHttpMessageConverter offers default Gson
configuration, but can be customized by setting an alternate Gson
instance.

RestTemplate will enable GsonHttpMessageConverter only if Jackson 2 is
not found on the class path, because by default GsonHttpMessageConverter
supports the same media types as Jackson.

Issue: ANDROID-155