set_directory cannot be used in combination with set_file_name on Mac

[TomokiMiyauci]

related with #35

FileDialog set_directory cannot be used in combination with set_file_name.

It behaves as if set_directory is not called.

use rfd::FileDialog;

fn main() {
    FileDialog::new()
        .set_directory("/Users/user/Desktop")
        .set_file_name("test.txt")
        .save_file();
}