Add a helper function used by the get, put, post, and delete client func...

...tions to merge either 'params' or 'data' request kwargs in with the payload.  Added/updated tests.  This change will make masquerading easier.
The first - if you could set up the structure being passed as a defaultdict(list), you can just assume that there's an empty list for all possible keys so you don't need the extra method.