DataURI without optional <mediatype> part incorrectly put inside single quote

[awarecan]

• clean-css version - npm ls clean-css: 4.2.3
• node.js version - node -v: 12.16.2
• operating system: Mac

Configuration options

var CleanCSS = require('clean-css');
var input = "@font-face{font-family:'B';src:url(\"data:;base64,SEVMTE8=\")}";
var output = new CleanCSS({}).minify(input);

Input CSS

@font-face {
    font-family: 'B';
    src: url("data:;base64,SEVMTE8=");
}

Actual output CSS

@font-face{font-family:'B';src:url("'data:;base64,SEVMTE8='")}

Expected output CSS

@font-face{font-family:'B';src:url(data:;base64,SEVMTE8=)}

Per RFC 2397, <mediatype> is optional in data uri. data:;base64 is valid, should not be put inside single quote.

It is a special case of #676