Fix test errors on OSX

On OSX, if LC_TIME and LC_CTYPE differs the output of strftime is not properly decoded
in Python 3. This makes sure that the 'utils.DateFormatter' and the related Jinja filter
'strftime' set the same value for LC_TIME and LC_CTYPE while formatting.

Also, '%a' is removed from DEFAULT_DATE_FORMAT in 'custom_locale' tests. OSX and *nix have
different conversions for '%a' ('Jeu' vs 'jeu.') and there is not a feasible way to handle
the difference for tests.