* 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