Interpolated, double-quoted variable as a part of selector produces an empty output

[saper]

Extracted from #1684 case "1.2":

$key: 'bar';
.test12#{'\\@#{$key}'} { content: '1.2'; }

produces empty string on libsass 3.3.2, while Ruby Sass gives

.test12\@bar {
  content: '1.2'; }

[mgreter]

Stripped down version:

foo {
    bar: #{'\\X'};
    baz: #{'\\#{X}'};
}

Libsass:

foo {
  bar: \X;
  baz: \\X; }

Sass:

foo {
  bar: \X;
  baz: \X; }

I wonder if this has changed over time on ruby sass or it may break other cases once I fix this one ...

[mgreter]

As expected this is one of the interpolation behaviors I don't get and I'm therefore unable to produce a fix that will not break anything else. It might have to do with a rule I've picked up somewhere, that quoted string can only be unquoted once, even if you call unquote multiple times. IMO we don't do that for interpolations. If I make the changes to make this spec pass all double interpolations with escapes seem to fail.