module Roda::RodaPlugins::MailProcessor::RequestMethods

def match_header((key, value))

value (which may be nil).
Match against a header specified by key with the given 
def match_header((key, value))
  return unless content = mail.header[key]
  if value.nil?
    @captures << content.decoded
  else
    _match_content(:header, value, content.decoded)
  end
end