Invalid reduction for keyframes (see YUI issue)

[jcward]

Suffers from an invalid 0% --> 0 minification in keyframes, like YUI (did/does): https://github.com/yui/yuicompressor/issues/151

> irb
2.2.5 :001 > require 'cssminify'
 => true 
2.2.5 :002 > CSSminify.compress('@keyframes foo { 0% { opacity: 0; } 100% {  opacity: 1; } }')
 => "@keyframes foo{0{opacity:0}100%{opacity:1}}" 

Inside @keyframes (and @-webkit-keyframes, etc) the % sign is necessary, 0% can't be reduced to 0.