Variables in at-directives should not be evaluated

[xzyfer]

Ref #1214

The content of generic at-directives should be a simple string constant. Currently we parse these values and evaluate them which is not how Ruby Sass behaves.

$foo: bar;
@keyframes $foo {
  from { a: b }
  to { a: c }
}

Ruby Sass

@keyframes $foo {
  from {
    a: b; }
  to {
    a: c; } }

Libsass

@keyframes bar{
  from {
    a: b; }
  to {
    a: c; } }

Spec added: sass/sass-spec#387

[mgreter]

We now have a passing spec test since #1249 was merged 🎈