Maps in @each with one variable should coerce into a list

[xzyfer]

Originally reported by @lunelson

https://twitter.com/lunelson/status/549157998409818112

.debug {
  @each $item in (a: 1, b: 2, c: 3) {
    each: $item;
  }
}

Expected output

.debug {
  each: a 1;
  each: b 2;
  each: c 3; }

Libsass output

.debug {
  each: a;
  each: b;
  each: c; }

http://sassmeister.com/gist/3c9367cc2bb2d645133f

Specs added sass/sass-spec#213