Colors should be case insensitive?

[asottile]

input.scss

a { b { color: lighten(Crimson, 10%); } }

Actual results

$ ./node_modules/.bin/node-sass --version
node-sass	4.5.3	(Wrapper)	[JavaScript]
libsass  	3.5.0.beta.2	(Sass Compiler)	[C/C++]
$ ./node_modules/.bin/node-sass test.scss test.scss  |& jq --raw-output .message
argument `$color` of `lighten($color, $amount)` must be a color

Backtrace:
	test.scss:1, in function `lighten`
	test.scss:1

Expected result

$ sass --version
Sass 3.5.1 (Bleeding Edge)
$ sass test.scss
a b {
  color: #ed365b; }

Originally reported in sass/libsass-python#214

[asottile]

Spec created: sass/sass-spec#1160

[asottile]

PR created: #2463