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.
Merge pull request #110 from seancribbs/rack-adapter-fix

Fix broken streaming in Rack adapter.