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.

There's a small chance somebody might stash something in the default provided "options" dictionary and it will be there for all future calls to serialize(), typically causing much confusion for the developer making the function call. A quick grep though the code does show empty dictionaries being used anywhere else as a default argument.