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.
I'm still a bit confused as I'm not appending to a list here, but rather merging into an existing nested dictionary or setting a new dictionary key-value pair.  I'm also uncertain about the comment where you're assuming no changes from the defaultdict comment... wouldn't all of the places where this method is being called have to change to accomodate your request?