Bugfix corpus christi

Thanks to @AndreMiras, a major bug was found in the ChristianMixin core class.

* the ``include_corpus_christi`` flag is now set to False by default,
* a test of this core module makes us sure that no religious holiday is included by default, except Christmas Day,
* a little cleanup ; the U.S.A. branch contained a "force-to-false" for the corpus christi flag.