- post-review fix regarding Item_fields
- added test for the changed name resolution


mysql-test/r/having.result:
  Test for changed name resolution of Item_fields
mysql-test/t/having.test:
  Test for changed name resolution of Item_fields
sql/item.cc:
  - Changed Item_field::fix_fields to perform the same name
    resolution as Item_ref::fix_fields because column references
    of subqueries inside HAVING may be represented as Item_fields,
    and they need to be resolved as Item_refs.
  - Adjusted Item_field::fix_fields so that it has the same variable
    names and structure as Item_ref::fix_fields.