Require that, at least, document.body exists before firing the ready event. This...

Require that, at least, document.body exists before firing the ready event. This may stop jQuery from working on pages that don't create a document.body (such as Safari, when no body exists) but considering that we've required a document.body since 1.3 and haven't broken any pages, this doesn't appear to be a pressing issue. Used the test case written by dmethvin to verify the bug and the fix. Fixes #4320.