Fix some minification errors related to zero value replacements

[lastikas]

Taking the following CSS @keyframe anim { 0% { opacity: 0; } 100% { opacity: 1; } } The output will be incorrectly rewritten to: @keyframe anim { 0 { opacity: 0; } 100% { opacity: 1; } }

This fix was introduced to YUI in pull request #231. I simply applied it to this port

For more information please visit: https://github.com/yui/yuicompressor/pull/231#issue-124952655