Extensions such as ".tar.gz" are now also supported. Before this patch,
ensure_ext('demo.tar.gz', '.tar.gz') would return 'demo.tar.tar.gz'.
This results in issues with the `ExportHelper` mix-in class; clicking
an existing file in the file dialogue warns about overwriting it
(highlighting the input box in red), but then saves to a different
file.
Also added a unit test for the new behaviour.
Reviewers: mont29, campbellbarton
Reviewed By: campbellbarton
Differential Revision: https://developer.blender.org/D1498
