Rack::Chunked and ContentLength middlewares by default
This is needed to match the behavior of Rack::Server for
RACK_ENV=(deployment|development), actually. This won't
affect users of other RACK_ENV values.
This change has minor performance consequences, so users
negatively affected should set RACK_ENV to "none" instead for
full control of their middleware stack.
This mainly affects Rainbows!/Zbatery users since they have
persistent connections and /need/ Content-Length or
Transfer-Encoding:chunked headers.