Add todo spec test for libsass issue 2818

sass/libsass#2818

Author: mgreter

spec/libsass-todo-issues/issue_2818.hrx

@@ -0,0 +1,9 @@
+<===> input.scss
+$map: ("lightness": 10%, "saturation": 10%);
+$base: call(get-function('scale-color'), #dedede, $map...);
+test { color: $base; }
+
+<===> output.css
+test {
+  color: #e4dede;
+}