core: Optimize the private sanitizeCssPropName() function
core: Optimize the private sanitizeCssPropName() function

HTMLElement#css() wasn't being optimized because of a problem with sanitizeCssPropName() which caused
that function to be deoptimized. This commit fixes that problem so now both sanitizeCssPropName() and
HTMLElement#css() are optimized and it also improves the minified and gzippped file size.

woollybogger/Firebolt,woollybogger/Firebolt