[BUGFIX beta] Use document.body.contains if Node is undefined.

Fixes the changes made in https://github.com/emberjs/ember.js/pull/5240
to work with IE8.

Closes https://github.com/emberjs/ember.js/issues/5250.
Closes https://github.com/emberjs/ember.js/pull/5256.
Merge pull request #5258 from rwjblue/handle-old-ie-node-undefined

[BUGFIX beta] Use document.body.contains if Node is undefined.