Fix unquote handling for non string types

Feature is deprecated in ruby sass. We may
should print a warning to inform the user.

Fixes sass#1258

functions.cpp

@@ -771,14 +771,17 @@ namespace Sass {
       if (dynamic_cast<Null*>(arg)) {
         return new (ctx.mem) Null(pstate);
       }
+      else if (List* list = dynamic_cast<List*>(arg)) {
+        return list;
+      }
       else if (String_Quoted* string_quoted = dynamic_cast<String_Quoted*>(arg)) {
         String_Constant* result = new (ctx.mem) String_Constant(pstate, string_quoted->value());
         // remember if the string was quoted (color tokens)
         result->sass_fix_1291(string_quoted->quote_mark() != 0);
         return result;
       }
       To_String to_string(&ctx);
-      return new (ctx.mem) String_Constant(pstate, unquote(string(arg->perform(&to_string))));
+      return new (ctx.mem) String_Constant(pstate, string(arg->perform(&to_string)));
     }
 
     Signature quote_sig = "quote($string)";