Handle :case-sensitive properly


If the first clause doesn't match due to mismatched case, it would
roll down to the second clause, causing case-differing strings to
erroneously match. By checking for case again, we can avoid this.

Note, however, that this does not fix the similar but unrelated
inconsistent usage of *unify-string-case-sensitive-p* and
*unify-string-case-insensitive-p* (the former being used in the code
and the latter appearing in documentation). That bug will remain
until the canonical form is decided upon.