WSGI conform list of (header, value) tuples.

def headerlist(self):
        """ WSGI conform list of (header, value) tuples. """
        out = []
        headers = list(self._headers.items())
        if 'Content-Type' not in self._headers:
            headers.append(('Content-Type', [self.default_content_type]))
        if self._status_code in self.bad_headers:
            bad_headers = self.bad_headers[self._status_code]
            headers = [h for h in headers if h[0] not in bad_headers]
        out += [(name, val) for (name, vals) in headers for val in vals]
        if self._cookies:
            for c in self._cookies.values():
                out.append(('Set-Cookie', _hval(c.OutputString())))
        if py3k:
            out = [(k, v.encode('utf8').decode('latin1')) for (k, v) in out]
        return out