Please describe the problem:
In earlier versions of gthumb, the Fit button scaled images to fill the screen regardless of their original sizes:  overly large images were scaled down, smaller images were scaled up.  2.10.6 doesn't scale up when the Fit button is clicked.

Steps to reproduce:
1. Select an image whose normal size is smaller than the screen
2. Select Fullscreen  note blank space surrounding image
3. Click Fit button - note that nothing happens


Actual results:
Small image remains unchanged, centered in blank fullscreen

Expected results:
Small image should be expanded to fit the window (appropriately, so that at least one dimension fills corresponding screen dimension).

Does this happen every time?
Yes

Other information:
The Shift-X keyboard shortcut does set fullscreen mode to operate properly, so it looks like a callback just got missed in coding the Fit button.