Resume writing.

        Successive calls to this method will fails unless
        :meth:`pause_writing` is called first.

def resume_writing(self, exc=None):
        '''Resume writing.

        Successive calls to this method will fails unless
        :meth:`pause_writing` is called first.
        '''
        assert self._paused
        self._paused = False
        waiter = self._waiter
        if waiter is not None:
            self._waiter = None
            if not waiter.done():
                if exc is None:
                    waiter.set_result(None)
                else:
                    waiter.set_exception(exc)
            self.transport.resume_reading()
        self._write_from_buffer()