Added test for optional

Author: lukeapage

test/less/import.less

@@ -2,6 +2,8 @@
 
 @import url(/absolute/something.css) screen and (color) and (max-width: 600px);
 
+@import (optional) "file-does-not-exist.does-not-exist";
+
 @var: 100px;
 @import url("//ha.com/file.css") (min-width:@var);