def headers(self, headers=None, **kw):
        """
        Defines a dictionary of arguments.

        Header keys are case insensitive.

        Arguments:
            headers (dict): headers to match.
            **headers (dict): headers to match as variadic keyword arguments.

        Returns:
            self: current Mock instance.
        """
        headers = kw if kw else headers
        self._request.headers = headers
        self.add_matcher(matcher('HeadersMatcher', headers))