Lua : fix StringBuf.add for special cases

std/lua/_std/StringBuf.hx

@@ -65,9 +65,17 @@ class StringBuf {
 		If `x` is null, the String "null" is appended.
 	**/
 	public inline function add<T>( x : T ) : Void {
-		var res = Std.string(x);
-		Table.insert(b, res);
-		length +=  res.length;
+		if (Std.is(x,String)){
+			Table.insert(b, x);
+			length +=  Std.string(x).length;
+		} else if  (Std.is(x,Int)){
+			Table.insert(b, x);
+			length++;
+		} else {
+			var str = Std.string(x);
+			Table.insert(b, str);
+			length += str.length;
+		}
 	}
 
 	/**