Send an option body with a POST request (enhanced)

[pablorodiz]

I was trying to use rg1220 changes to the original library so I could send a body in a post request, but I need additional headers and so I need to use beginRequest() and endRequest(). I noticed that in the rg1220 version body was not sent in these conditions because finishHeaders() was being called always with NULL as parameters from endRequest(). So I added two variables to store pointers to the body and content type char arrays so they can be used later in the case finishHeaders() is called later from outside startRequest.