Fixed enrich_date.convert_dates to allow begin and end with None

This was preventing sourceResource.date from being assigned as
expected (with `begin` and `end` set to None) when given
undiscernable dates.
Changed test to allow for stripped period in "19th c."

Changed test_enrich_date.test_enrich_date_parse_century_date() to
allow incoming "19th c." to get set as "19th c" because of the
way we're trying to use utilities.remove_brackets_and_strip() to
strip this field in the interest of code reuse.  It's easier this
way.  If preserving the period is an issue, then that could be
addressed in the aforementioned function.