keydown event is not reset correctly

For some reason this happens only with modifier keys.

```
$ xdotool keydown Super_L
```
now try pressing Super_L+something, then release it. Super_L get disabled as expected, but...

typing above command again will do nothing, I have to do before:
```
$ xdotool keyup Super_L
```
even `--clearmodifiers` doesn't work.

Is this expected behaviour?