Fix #14432: Always return string from .css("z-index"). Close gh-1395.
No ticket: Small curCSS size optimizations
No ticket: Test cleanup
Preparations for npm/bower publishing
Fix #14394: Changing style !important in webkit. Close gh-1385.
No ticket: Clean up uglifyjs compress options. Close gh-1382.
(cherry picked from commit e15627022f0c5e13a972baffb41f6f52ca4d3ec3)
Only set uglify options if not already the default