Problem: Item_copy did not set "fixed", which resulted in DBUG_ASSERT in some cases.
Fix: adding  initialization of the "fixed" member

Adding tests:
  mysql-test/include/ctype_numconv.inc
  mysql-test/r/ctype_binary.result
  mysql-test/r/ctype_cp1251.result
  mysql-test/r/ctype_latin1.result
  mysql-test/r/ctype_ucs.result

Adding initialization of the "fixed" member:
  sql/item.h