docs: def iter_headers(self):
          ''' Yield (header, value) tuples, skipping headers that are not
              allowed with the current response status code. '''
          headers = self._headers.items()
          bad_headers = self.bad_headers.get(self._status_code)
          if bad_headers:
              headers = [h for h in headers if h[0] not in bad_headers]
          for name, values in headers:
              for value in values:
                  yield name, value
          if self._cookies:
              for c in self._cookies.values():
                  yield 'Set-Cookie', c.OutputString()