fix Issue #676 - "explicit" nulls within RuntimeTypeAdapterFactory

[lcoote]

Using custom TypeAdapters, it is possible to force null values within a
particular type to be explicitly serialized, by overriding the
setSerializeNulls setting on the supplied JsonWriter.

However, this behavior fails in the context of a
RuntimeTypeAdapterFactory because a second JsonTreeWriter is used and
settings are not propagated from one to the other.

This change fixes this by:

• propagating the original setSerializeNulls setting to the created
  JsonTreeWriter
• temporarily forcing the setSerializeNulls setting to true on the
  oriiginal JsonWriter to ensure any explicit nulls are preserved.

Issue #676

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[lcoote]

I signed it!

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