fixed box-sizing default assumptions

[rlauck]

The default box-sizing is content-box, not border-box. This causes a layout issue in IE 10 and probably others when no box-sizing is set and $.browser.boxModel is not available. Honestly, there is no reason to even make all those browser version checks anyway. Just fetch the boxSizing value and if its not available or not set, default to content-box.

See: https://developer.mozilla.org/en-US/docs/Web/CSS/box-sizing

[allpro]

"Just fetch the boxSizing value and if its not available or not set, default to content-box."

This is incorrect for IE6 & IE7 quirks-mode, which work like box-size=border-box, yet do not support box-size CSS. If all other browsers supported box-size, I would enable border-box in the hard-coded CSS to simplify calculations, but some do not support it so still need to test.

I'll investigate the IE10+ issue you mention and fix as needed. I'll leave this PR open until resolved.