optimizeColors: rgba|hsla with opacity >=1.0 should change to rgb|hsl

[clshortfuse]

Environment

• clean-css version - 4.2.1:
• node.js version - 10.16.0:
• operating system: Windows 10 / Ubuntu 18

Configuration options

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

Input CSS

.a { color: rgba(255, 255, 255, 1); }
.b { color: hsla(0, 100%, 50%, 1); }
.c { color: RGB(255, 255, 255); }
.d { color: HSL(0, 100%, 50%); }

Actual output CSS

.a{color:rgba(255,255,255,1)}.b{color:hsla(0,100%,50%,1)}.c{color:RGB(255,255,255)}.d{color:HSL(0,100%,50%)}

Expected output CSS

.a{color:#fff}.b{color:red}.c{color:#fff}.d{color:red}

I also noticed that color parsing is case-sensitive when they shouldn't be. It's messing up some SCSS code I'm using. In SCSS, rgb() is an internal function so I use RGB() instead. clean-css doesn't pick it up. PR coming up.