Call to check_plain in render() function results in double-escaped chars in box titles

Box titles are being double-escaped before they're output. I can only find one call to check_plain, in the render() method of boxes_simple - I'm not sure where else check_plain or similar is being called on the title (I think it's happening after the render() function is called. The point is, without the check_plain, the chars are being escaped anyway, so adding it here just results in things like it's instead of it's.
Patch to follow...