Fix set_caption(None) oddness on xlib (seen in issue 282, though not the main issue).
Fix set_caption(None) oddness on xlib (seen in issue 282, though not the main
issue).

niklaskorz/pyglet,seeminglee/pyglet64,niklaskorz/pyglet,adamlwgriffiths/Pyglet,adamlwgriffiths/Pyglet,adamlwgriffiths/Pyglet,adamlwgriffiths/Pyglet,niklaskorz/pyglet,seeminglee/pyglet64,seeminglee/pyglet64,niklaskorz/pyglet