removeUnusedAtRules option drops @font-face if the usage has !important

[alfaproject]

Environment

• clean-css version: 4.1.9
• node.js version: 8.9.1
• operating system: macOS 10.13.1

Configuration options

var CleanCSS = require('clean-css');
new CleanCSS({
    level: {
        2: {
            removeUnusedAtRules: true,
        }
    },
});

Input CSS

@font-face {
  font-family: f;
}

i {
  font-family: f !important;
}

Actual output CSS

i{font-family:f!important}

Expected output CSS

@font-face{font-family:f}i{font-family:f!important}

[jakubpawlowicz]

It's fixed in 4.1.10.