Removed a mutable default param in Serializer.serialize().

I haven't seen this pop up explicitly as a bug in code; only noticed it
through code inspection.
Merge pull request #286 from malcolmt/mutable-default-fix

Removed a mutable default param in Serializer.serialize().
Adding support for partial response! This will allow users of the API to specify what fields they need in response to API call. Adding test cases and updating the documentation to reflect the same.