Error message mismatch for duplicate map key

[saper]

Difference between Ruby Sass and libsass from sass/sass-spec#373

$a: 'foo';
$b: 'foo';
$map: (
  $a: 1,
  $b: 2
);

-"Duplicate key "foo" in map ($a: 1, $b: 2)."
+"Duplicate key "foo" in map ("foo": 2)."

Specs added https://github.com/sass/sass-spec/pull/373/files