* convert_content_type! is handled by assign_default_content_type_and_charset!
* set_content_length! should be handled by the endpoint server. Otherwise
each middleware that modifies the body has to do the expensive work of
recalculating content_length.
* convert_language! appears to be legacy. There are no tests for this
* convert_cookies! should be handled by the new HeaderHash in Rack
* Use an integer for .status's internal representation to avoid needing to
do String manipulation just to find out the status
