BUG, TST: Fix pandas.core.strings.str_contains when handling regex=False and case=False

`pandas.core.strings.str_contains` does not match in a case insensitive fashion *at all* when given `regex=False` and `case=False`.  This PR should fix the situation.  Additionally, there is test coverage for `pandas.core.strings.str_contains` case insensitive matching both with and without regular expressions enabled.