Problem: Item_func_hex::val_str() returned data in ASCII format,
which did not match collation.collation pointing to my_charset_utf32_general_ci.
Fix: changing parent class of Item_func_hex to Item_str_ascii_func,
as val_str() implementation is heavily ASCII-oriented.

  mysql-test/r/ctype_utf32.result
  mysql-test/t/ctype_utf32.test
  Adding test case


  sql/item_strfunc.cc
  sql/item_strfunc.h
  - Changing parent class to Item_str_ascii_func
  - Clean-up in Item_func_hex::fix_length_and_dec()
    Using fix_char_length() instead of setting max_length directly.