Support multipart POST

The current code always assumes that the request data is a string (in order to determine the content length). However if you use cURL and want to do a multipart post then the request data must be an array of post fields. Aboviously this will result in a warning being generated due to the string length function being called on the request data.

I attached a patch which checks if the $request->data is a string before determining the content length.