Since 06ab7b27,
`GCSServiceTest#test_signed_URL_response_headers` is broken.
https://travis-ci.org/rails/rails/jobs/460454477#L7084-L7087

This seems to be due to lack of `content_type` at upload.
This is solved by specifying `conten_type`.

However, since the same content is also tested with `test_upload_with_content_type`,
it will be duplicated content, so I think that can remove `test_signed_URL_response_headers`.