Allow upload of bitmaps by changing Content-Type to "multipart/form-data"

Since I had to upload bitmaps to an existing backend and I was using the DataDroid library in the project, it would be great if an image upload would be possible by the DataDroid library as well.

For my bitmap upload I had to use the Content-Type "multipart/form-data". I therefore extended the NetworkConnection[Impl] to allow setting of bitmaps to upload. After settings those, the Content-Type is set to "multipart/form-data" and all other parameters will be sent using this Content-Type, too.

P.S.: Sorry for the many whitespace changes. My eclipse totally messed it up :(