Fix for issue #10 (AttributeError: NoneType for lang)

[jm9e]

In case translation.deactivate_all() from django.utils has been called, translation.get_language() returns None. This caused an error in the i18n_l10n_prefix method.

This pull request contains a test which examines exactly this case, testing the error case and the correctness of the returned prefix for that case. It also contains a simple fix for this problem.

[zsoldosp]

@jm9e looks good to me too. Could you please bump the version number to 0.1.5 & add the release notes (or should we do that after merging, @kairichard)?

minor: IMO it would be more correct to use return '.'.join(p for p in parts if p) (which we didn't do originally 'coz it didn't occur to us that lang/locale can be None ☹️), but the current approach does fix a bug, which is more important than the exact cache key used, so I wouldn't block the release for this

[kairichard]

@jm9e @zsoldosp Let's add release notes to this branch and then merge it. We did that with the previous ones too.

[jm9e]

@zsoldosp @kairichard Thanks for your feedback. I updated the version and added release notes as requested.

[zsoldosp]

@jm9e thanks for the fix, and the new release is up on pypi!