Update Element.Dimensions.js

Fix "Unspecified Error" in Internet Explorer when calling getSize() on a node that isn't in the DOM.

cf. http://bugs.jquery.com/ticket/4996 , https://github.com/jquery/jquery/commit/cf672a2e7a886cac5ae62f6772c6b4b43b19a2fc , and https://github.com/jquery/jquery/commit/ec7ea3fba15379ebe8ddff5f6c99ec3faf8d6d17
Merge pull request #2648 from barryvan/patch-2

Update Element.Dimensions.js