Fix document.body check

[seagullua]

trafficstars

We have noticed that the current body check does not work properly all the times. We have replaced it with jQuery inspired check which will wait until document.body can be accessed.

[pandamicro]

Can you provide a step by step instruction to reproduce the issue ? Normally, if document.body can't be accessed, it will wait until window.load, I believe body should be accessible by then