TIMESTAMP.

Item_cache::get_cache wasn't treating TIMESTAMP as a DATETIME value thus
returning string cache for items with TIMESTAMP type. This led to incorrect
TIMESTAMP -> INT conversion and to a wrong query result.

Fixed by using Item::is_datetime function to check for DATETIME type group.


mysql-test/r/type_timestamp.result:
  Added a test case for the bug#59330.
mysql-test/t/type_timestamp.test:
  Added a test case for the bug#59330.
sql/item.cc:
  Bug#59330: Incorrect result when comparing an aggregate function with
  TIMESTAMP.
  Item_cache::get_cache now uses is_datetime member function to detect DATETIME
  type group.