* only copy args[0] to args[2] after fix_fields (when all item
  substitutions have already happened)

* change QT_ITEM_FUNC_NULLIF_TO_CASE (that allows to print NULLIF
  as CASE) to QT_ITEM_ORIGINAL_FUNC_NULLIF (that prohibits it).
  So that NULLIF-to-CASE is allowed by default and only disabled
  explicitly for SHOW VIEW|FUNCTION|PROCEDURE and mysql_make_view.
  By default it is allowed (in particular in error messages and
  debug output, that can happen anytime before or after optimizer).