Yield (header, value) tuples, skipping headers that are not
            allowed with the current response status code.

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()