Merge pull request #2 from unlooped/master

check if the content-type is set before calling "split" on it.
Fixes issue with GET and no data passed in, where `?` would get appended on the URL
HEAD should be treated like GET, except its response has empty body
Ability to remove header fields when passing `null` as value to the `header` method (`.header('X-PINGPONG', null)`)
Added OPTIONS verb
Merge pull request #3 from w00fz/fixes

Various fixes and small enhancements
[REF] don't set an empty function as callback, just don't call it
[REF] send the request also when no callback is set.