Well, I was going to say that resolutions are absolute and I remember being able to do that a long time ago.
But, I just did it and it does stretch to fill the screen. So, it is either a setting with your monitor, or drivers.
Is this a laptop or desktop.... laptop you may be able to hit Fn+F7 or F8.
Desktop, the setting (if it exists) will be in the drivers, or monitor options,,, maybe.
Technically,,, I think it should do it by default, unless your video card and monitor are not capable. But I have no idea.