fixes #11004: getWH incorrectly removes padding and border width when box-sizing is border-box

[mikesherov]

.css('width') should respect box-sizing, where as .width() should always return the content-width, as defined by the API. Here is a perf showing that it's a tiny drop in performance for .css('width') except when box-sizing is border-box because in that case, .css('width') is equivalent to offsetWidth.

It's not 30 bytes, like @timmywil asked for, but it's close enough...

jQuery Size - compared to last make
  252107   (+860) jquery.js
   93724   (+134) jquery.min.js
   33304    (+44) jquery.min.js.gz

[dmethvin]

Landed in 1.8pre: d7217cc