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).

gdkar/pyglet,odyaka341/pyglet,gdkar/pyglet,qbektrix/pyglet,xshotD/pyglet,cledio66/pyglet,kmonsoor/pyglet,qbektrix/pyglet,mpasternak/michaldtz-fixes-518-522,mpasternak/michaldtz-fixes-518-522,odyaka341/pyglet,mpasternak/pyglet-fix-issue-518-522,mpasternak/pyglet-fix-issue-518-522,mpasternak/michaldtz-fix-552,Alwnikrotikz/pyglet,google-code-export/pyglet,shaileshgoogler/pyglet,cledio66/pyglet,mpasternak/pyglet-fix-issue-552,google-code-export/pyglet,kmonsoor/pyglet,qbektrix/pyglet,arifgursel/pyglet,Austin503/pyglet,kmonsoor/pyglet,shaileshgoogler/pyglet,cledio66/pyglet,mpasternak/michaldtz-fixes-518-522,Alwnikrotikz/pyglet,qbektrix/pyglet,mpasternak/pyglet-fix-issue-518-522,cledio66/pyglet,gdkar/pyglet,xshotD/pyglet,mpasternak/pyglet-fix-issue-552,arifgursel/pyglet,gdkar/pyglet,mpasternak/pyglet-fix-issue-552,Alwnikrotikz/pyglet,qbektrix/pyglet,google-code-export/pyglet,xshotD/pyglet,arifgursel/pyglet,xshotD/pyglet,shaileshgoogler/pyglet,mpasternak/michaldtz-fixes-518-522,Austin503/pyglet,cledio66/pyglet,Austin503/pyglet,arifgursel/pyglet,Alwnikrotikz/pyglet,xshotD/pyglet,mpasternak/michaldtz-fix-552,mpasternak/michaldtz-fix-552,Austin503/pyglet,kmonsoor/pyglet,Austin503/pyglet,google-code-export/pyglet,mpasternak/michaldtz-fix-552,Alwnikrotikz/pyglet,kmonsoor/pyglet,odyaka341/pyglet,mpasternak/pyglet-fix-issue-518-522,arifgursel/pyglet,mpasternak/pyglet-fix-issue-552,odyaka341/pyglet,shaileshgoogler/pyglet,odyaka341/pyglet,google-code-export/pyglet,gdkar/pyglet,shaileshgoogler/pyglet