more consistent popover sizing behavior when an explicit preferredContentSize isn't set

There's currently a bug currently in Trello where the "move all cards in list" resizes to CGSizeMake(0, 44), because that's the contentSizeForViewInPopover of the navigation controller. Although preferredContentSize does the right thing in iOS 7, contentSizeForViewInPopover doesn't, and it tries to fall back on that if you don't set an explicit preferredContentSize.