Add setSerializeNulls() to JsonWriter, so nulls can be skipped from serialization. This does not yet impact JsonElementWriter.
One change in behavior: if the only value is skipped, we now emit "null" rather than "".
GSON 1.x applies different rules for versioning for classes vs fields. So, if you deserialize a
JSON into a field that is supposed to be skipped, the field is set to null (or default value).
However, if you deserialize it to a top-level class, a default instance is returned.
GSON 2.x returns null for the top-level class.
Previously we would set the field if the created instance is being assigned to a field of another object. We wouldn't set it when the created instance is a collection element or the top-level object.
Got rid of default Hierarchy Serializer/Deserializers from DefaultTypeAdapters.
Got rid of methods for gettting default serializers/deserializers/instance creators. Instead we reuse the static final instances.
Fixed warnings in TypeAdapters where a parameterized type T was hiding the parameterized T in methods.
Removed support to unwrap single element array of enums into enum values. Also removed the test that verifies this behavior.
