Problem: separator was not converted to the result character set,
so the result was a mixture of two different character sets,
which was especially bad for UCS2.
Fix: convert separator to the result character set.


mysql-test/r/ctype_ucs.result:
  Adding test case
mysql-test/r/ctype_ucs2_def.result:
  Adding test case
mysql-test/t/ctype_ucs.test:
  Adding test case
mysql-test/t/ctype_ucs2_def.test:
  Adding test case
sql/item_sum.cc:
  Adding conversion of separator to the result character set
sql/sql_yacc.yy:
  Fixing GROUPC_CONCAT problems when "mysqld --default-character-set=ucs2".