The list of headers for this response

def _get_headers(self, environ):
        """The list of headers for this response
        """
        headers = self.headers
        method = environ['REQUEST_METHOD']

        if has_empty_content(self.status_code, method) and method != HEAD:
            headers.pop('content-type', None)
            headers.pop('content-length', None)
            self._content = ()
        else:
            if not self.is_streamed():
                cl = reduce(count_len, self._content, 0)
                headers['content-length'] = str(cl)
            ct = headers.get('content-type')
            # content type encoding available
            if self.encoding:
                ct = ct or 'text/plain'
                if ';' not in ct:
                    ct = '%s; charset=%s' % (ct, self.encoding)
            if ct:
                headers['content-type'] = ct
            if method == HEAD:
                self._content = ()
        # Cookies
        if (self.status_code < 400 and self._can_store_cookies and
                self._cookies):
            for c in self.cookies.values():
                headers.add('set-cookie', c.OutputString())
        return headers.items()