named colours behave oddly when used in variables

There's something odd going on with named colours. They're meant to be "indistinguishable from unqoted strings".

```
@function is_gold($c) {
    @if ($c == gold) {
        @return 'yes';
    }
    @return 'no';
}

div {
    foo: is_gold(gold);
    bar: is_gold(white);
}
```

sass gives me:

```
div {
  foo: "yes";
  bar: "no"; }
```

sassc gives me:

```
div {
  foo: 'no';
  bar: 'no'; }
```


