LAST_DAY() says it returns a DATE, not a DATETIME, but didn't zero the time fields.
Adapted from a patch kindly supplied by Claudio Cherubino.


mysql-test/r/func_time.result:
  show that LAST_DAY() returns only a DATE, not a DATETIME
mysql-test/t/func_time.test:
  show that LAST_DAY() returns only a DATE, not a DATETIME
sql/item_timefunc.cc:
  zero time-fields as we return only a DATE