give a warning if the body position isn't static

If you set the body tag position to absolute or fixed and the height to 100% of a child iframe, the iframe will not resize. If you set the height to a percentage that is less than 100%, the iframe shrinks every resize.
 
[demo](https://github.com/zippy1981/pym.js-body-bug-demo).

I think this deserves a console.warn. I'm open to other ideas on how to deal with this behavior.