Override Rack::Request#media_type instead of #content_type

Rack::Request#content_type is used by a lot of Rack::Request methods
like #media_type_params and #content_charset. All these methods
expect #content_type to be the full env['CONTENT_TYPE'] header.

Since we are overriding #content_type to return only the Mime::Type.to_s
these methods stop to work since no information about media type
parameters are returned on #content_type.