more complete change for both iOS 5 & iOS 6

previous pull request for this actually was doing the right thing to differentiate between iOS5 and iOS6 by relying on whether
respondsToSelector:@selector(shouldAutorotateToInterfaceOrientation:)

in the code that you modified after accepting the pull request, it stopped working.

i have tried to re-fashion this pull-request so the logic is clearer about what's going on.  in terms of logic, it's almost exactly the same as the previous pull-request, but it's been unwound a bit so you can tell what is iOS5 based and what is iOS6 based.