docs: def get_header(self, hdrclass, returnval=None):
          '''
          Return the first header object that is of
          class hdrclass, or None if the header class isn't
          found.
          '''
          if isinstance(hdrclass, str):
              return self.get_header_by_name(hdrclass)
  
          for hdr in self._headers:
              if isinstance(hdr, hdrclass):
                  return hdr
          return returnval