for sailthru/sailthru-python-client#40 to better check the json where ne...

...eded

Also adding tests to deal with invalid json, which is what the JSONDecoder is supposed to be checking.
The call to get_send returns a response object; the get_body() call returns a dictionary and not a string.
You should be using json.JSONEncoder.encode(send_response). 

Although looking at how this function works altogether, it can be simplified a lot by not even converting to a JSON string object.