Make copies of default values to TPayload objects

[mrterry]

If the same object is used across TPayload objects, mutating that field will affect subsequent TPayload object instances.

closes #261

[mrterry]

@lxyu I've added a test to make it explicit that TPayload.init must support positional args. TException is called several times with only positional args. Elsewhere in the code TProtocolException, TDecodeException, TApplicationException, and TTransportException are called similarly.