Problem: CASE didn't work with a mixture of different character
sets in THEN/ELSE in some cases.
This happened because after character set aggregation
newly created Item_func_conv_charset items corresponding
to THEN/ELSE arguments were not put back to args[] array.

Fix:
put all Item_func_conv_charset back to args[].


  @ mysql-test/include/ctype_numconv.inc
  @ mysql-test/r/ctype_ucs.result
  Adding tests

  @ sql/item_cmpfunc.cc
  Put "agg" back to args[] after character set aggregation.