Font-variation-settings getting quotes stripped & shouldn't

[jpamental]

I've found that the new font-variation-settings CSS attribute is getting quotes stripped out when it should not (it should work just like font-feature-settings, which are handled properly). I found the line and included a change below.

Precheck

• Do a quick search and make sure a bug has not yet been reported;
• do a quick check if the bug still exists in the latest patch version;
• finally, be nice and have fun!

Environment

• clean-css version - npm ls clean-css: v4.1.11
• node.js version - node -v: v8.11.3
• operating system: Mac OS High Sierra

Configuration options

var CleanCSS = require('clean-css');
new CleanCSS({
  // compatibility: 'ie8',
  // format: 'keep-breaks',
  // level: 2
})

Input CSS

h2 {
 font-variation-settings: 'wght' 600 'opz' 48;
}

Actual output CSS

h2 {
 font-variation-settings: wght 600 opz 48;
}

Expected output CSS

h2 {
 font-variation-settings: 'wght' 600 'opz' 48;
}

I found the line in /lib/optimizer/level-1/optimize.js, line 337:
if (name == 'content' || name.indexOf('font-feature-settings') > -1 ||

if changed to this:

if (name == 'content' || name.indexOf('font-variation-settings') > -1 || name.indexOf('font-feature-settings') > -1 ||

fixes the issue