setMultipartFile improvements

Add optional content-type header.
Automatically detect content-type.
Fix NPE when setting after calling configure().userAgent(String).
Add setMultipartParameters method.
Merge branch 'master' of https://github.com/koush/ion

# By Koushik Dutta
# Via Koushik Dutta
* 'master' of https://github.com/koush/ion:
  Add setMultipartParameters method.
  Fix NPE when setting after calling configure().userAgent(String).
  setMultipartFile improvements