Update WYPopoverController.m
Update WYPopoverController.m

Fix issue #47
window may has a background subview while it's hidden, that cause the popover can not be shown up.
pinkowitz/WYPopoverController,huntit/WYPopoverController,lijie121210/WYPopoverController,Mindera/WYPopoverController,thelearningclinic/WYPopoverController,vitalys/WYPopoverController,Kingside/WYPopoverController,xswm1123/WYPopoverController,Voxer/WYPopoverController,Medium/WYPopoverController,brittonmobile/WYPopoverController,dannys42/WYPopoverController,nicolaschengdev/WYPopoverController,iv-mexx/WYPopoverController,sammcewan/WYPopoverController,jiamaozheng/WYPopoverController,netcosports/WYPopoverController,orta/WYPopoverController,appone/WYPopoverController,CODE4APPLE/WYPopoverController,Eveian/WYPopoverController,richiewoo/WYPopoverController,blackpixel/WYPopoverController