Adding test case.
item_func.cc:
  Bug#13751 find_in_set: Illegal mix of collations.
  Character set conversion was forgotten in find_in_set.


sql/item_func.cc:
  Bug#13751 find_in_set: Illegal mix of collations.
  Character set conversion was forgotten in find_in_set.
mysql-test/t/ctype_utf8.test:
  Adding test case.
mysql-test/r/ctype_utf8.result:
  Adding test case.