Added support for NSData and made UNIRest defaultHeaders take precedence over UNIRests built in default headers (they were merged before).

In our app we needed support to send images in NSData format instead of pointers with NSUrl. Added native support for that and also made it so that the Unirest built-in default headers are overridden by the user defined [UNIRest defaultHeaders]. Both of these changes can come in handy for other people too :)