Fix bug in max_length calculation for CountryField with multiple=True

[cnschn]

Fixes #469.

I'm not sure if there is a better way to test this - the override_settings decorator does not affect the test models, so I just instantiated a CountryField directly. Also deling the countries.countries in a test inside the TestCountryMultiple messes up other tests in the class, so I added a new one with setUp and tearDown to make sure the test would fail without the fix.