Clean up #add_content_type!
Clean up #add_content_type!

can't just do `headers.to_h`, since `#to_h` is not available for `nil`
in 1.9.x.

kidaa/travis-core,kidaa/travis-core,Tiger66639/travis-core,travis-ci/travis-core,developerfm/travis-core,developerfm/travis-core,travis-ci/travis-core,jantman/travis-core,GistIcon/travis-core,GistIcon/travis-core,final-ci/travis-core,Tiger66639/travis-core,travis-ci/travis-core,GistIcon/travis-core,jantman/travis-core,final-ci/travis-core,developerfm/travis-core,Tiger66639/travis-core,kidaa/travis-core,final-ci/travis-core