add Plack ::Utils and use the util for no entity body

May as well use the Plack::Util for this, that way if stuff changes someday you don't need to worry
Remove pointless return of $response

You don't need to return the changed response, the middleware callback will ignore it anyway.  The only time you return from the response_cb is when you want to return a body content callback filter.
Merge pull request #1 from jjn1056/master

minor changes, nothing big!