'Prepares the message for serialization by forcing the ``message``
and ``extra_tags`` to unicode in case they are lazy translations.
Known "safe" types (None, int, etc.) are not converted (see Django\'s
``force_unicode`` implementation for details).'

def _prepare(self):

    self.message = force_unicode(self.message, strings_only=True)
    self.extra_tags = force_unicode(self.extra_tags, strings_only=True)
