Improve CountryFilter to play nice with multiple=True

[pfouque]

The current implementation of the admin CountryFilter doesn't works well when is_multiple=True.
It filters when there is only one value (strict equality) instead of using __contains.

(This version handles that but not the possibility to search with multiple countries)

[pfouque]

I just noticed the "need-tests" label was added...
I added them, based on a variation of the existing ones. I hope it will be good enough
Note that I used AssertQuerySetEqual (instead of assertEqual) to ignore the ordering