Added ability to pass NSData directly into POSTs

In my implementations, I have found myself converting from NSData into NSString so it can be used with JSONModel when JSONModel just ends up turning it back into NSData later on.

NSData containing null characters cannot be converted into NSStrings because C-strings use the null character to signal the end of the string.

This addresses inefficiency when converting back-and-forth, but also makes it possible to POST data containing null characters.