Don't rely on Rack::Response for sanitizing response headers.

Rack::Response iterates over given body which in some cases,
like Fiber streaming, may block responding to client.
https://github.com/rack/rack/blob/master/lib/rack/response.rb#L35-L38

RackResponse is introduced instead to cleanup Content-Type header
in order to pass Rack::Lint.
Use reel on platform ruby_20 as well.

Conflicts:
	Gemfile
Also test ruby 2.0.0 on travis-ci.
Fix deprecation warning on ruby 2.0.

"Enumerator.new without a block is deprecated; use Object#to_enum"