Fix issues with deserializing mappings

We need to make sure that we can deserialize the interfaces on the
RootObjectMapping, which requires specifying ResolveAsTypeConverter
on all the appropriate interfaces. I added a test as well, though I'm
not sure it's in the right place, and I added a Deserialize function
on TestElasticClient to make it easier to roundtrip the serialization.

I'm not sure if that was the right place to put the test, or the right approach, but it seemed like a good idea to have tests for whether we can serialize/deserialize RootObjectMapping.