Fixed #9533 - Position: positioning within document raises error

Bug #9533

Added a unit test raising the error & added an isDoc boolean to withinInfo to avoid calling css( "overflow-x" ) on document.

Note that the error isn't raised when css( "overflow-x" ) is called on another element earlier. So the test may not fail when running all unit tests. It does fail when only running 'tests/unit/position/position.html'

http://bugs.jqueryui.com/ticket/9533