Add respond with empty image

As discussed at RubyConf
Closes #86 
All credit should go for @parolkar I just implemented his changes.
Could you test to check response body?
Based on `test_json` and check `res.body` with EMPTY_GIF_IMAGE value.

https://github.com/fluent/fluentd/blob/25b96ef41916409114a3abf1a2db255152c5fdd7/test/plugin/test_in_http.rb#L48