Overrides default start behaviour by raising ConnectionError instead
        of custom requests_mock.exceptions.NoMockAddress.

def start(self):
        """Overrides default start behaviour by raising ConnectionError instead
        of custom requests_mock.exceptions.NoMockAddress.
        """
        if self._http_last_send is not None:
            raise RuntimeError('HttpMock has already been started')

        # 1) save request.Session.send in self._last_send
        # 2) replace request.Session.send with MockerCore send function
        super(HttpMock, self).start()

        # 3) save MockerCore send function in self._http_last_send
        # 4) replace request.Session.send with HttpMock send function
        self._patch_last_send()