minor : prevent Std.string to be called in stringRec

std/js/Boot.hx

@@ -72,7 +72,7 @@ class Boot {
 	}
 
 	@:feature("has_enum")
-	private static function __string_rec(o,s) {
+	private static function __string_rec(o,s:String) {
 		untyped {
 			if( o == null )
 			    return "null";