Fix '@arguments' behaviour, Closes #632

`.mixin` & `.mixin()` had differing behaviours.

test/css/mixins-args.css

@@ -59,6 +59,9 @@ body {
 .arguments2 {
   border: 0px;
 }
+.arguments3 {
+  border: 0px;
+}
 .edge-case {
   border: "{";
 }

test/less/mixins-args.less

@@ -116,6 +116,9 @@ body {
 .arguments2 {
     .mixin-arguments();
 }
+.arguments3 {
+    .mixin-arguments;
+}
 
 // Edge cases