fixed thumbnail frame not being displayed for some files

Currently:
![without_frame](https://f.cloud.github.com/assets/2727288/1021605/4094422a-0d13-11e3-9adb-03a3c143cb56.png)

After this fix:
![with_frame](https://f.cloud.github.com/assets/2727288/1021608/702a1424-0d13-11e3-8d22-4b25a796edda.png)

(Ported back from https://git.gnome.org/browse/nautilus/commit/?h=gnome-3-6&id=a83f876687a9f5704d64a93f7ea5c27781b43baf)