Problem: in case of string CASE/WHEN arguments with different
character sets, Item_func_case::find_item() called comparator
cmp_items[x] on mixed character set Items, so a 8-bit value could
be errouneously referenced to as being utf16/utf32 value,
which led to crash on DBUG_ASSERT() because of wrong value length.
This was wrong, as string comparator expects arguments in the same
character set.

Fix: modify Item_func_case's argument list after calling
agg_arg_charsets_for_comparison() - put the Items in "agg" array
back to "args", because some of the Items in the "agg" array might
have been changed to character set converters:
- to Item_func_conv_charset for non-constant items
- to Item_string for constant items

In other words, perform the same substitution which is done in
all other operations string comparison or string result operations:

Replace
  CASE         latin1_item              WHEN utf16_item THEN ... END
to
  CASE CONVERT(latin1_item USING utf16) WHEN utf16_item THEN ... END

Replace
  CASE utf16_item WHEN         latin1_item              THEN ... END
to
  CASE utf16_item WHEN CONVERT(latin1_item USING utf16) THEN ... END


  @ mysql-test/r/ctype_utf16.result
  @ mysql-test/r/ctype_utf32.result
  @ mysql-test/t/ctype_utf16.test
  @ mysql-test/t/ctype_utf32.test
  Adding tests

  @ sql/item_cmpfunc.cc
  Put "agg" back to "args".

  @ sql/sql_string.cc
  Backporting a fix for String::set_or_copy_aligned() from 5.6,
  for better test coverage:
  "SELECT _utf16 0x61" should expand the string to 0x0061 rather
  than to 0x000061.
  This fix was made in 5.6 under terms of "WL#4616 Implement UTF16-LE".