Add contentType to BinaryAttachment which will be used to send along with POST requests if it's not null.

The issue was that if there is no content type specified in POST body, then Facebook cannot recogonize it. Esp when you send a picture in private message, the picture will be showed as a file in attachment instead of rendering in the conversation directly.
Merge branch 'master' of https://github.com/UglyTroLL/restfb

# By Zhibo Wei
# Via Zhibo Wei
* 'master' of https://github.com/UglyTroLL/restfb:
  Add contentType to BinaryAttachment which will be used to send along with POST requests if it's not null.
Formatting tweaks for pull request