Sets Content-Type for `jbuilder` responses
Sets Content-Type for `jbuilder` responses

Similar to
<https://github.com/sinatra/sinatra/blob/1ac65a4089a5e247067cd0934408319430ee52e0/lib/sinatra/base.rb#L758>

linchus/tilt-jbuilder,nazipov/tilt-jbuilder,joneslee85/lotus-jbuilder,anthonator/tilt-jbuilder