Added  a test case for bug #11385.
group_by.test:
  Added  a test case for bug #11385.
field.h:
  Fixed bug #11385.
  The bug was due to not defined method decimals for the class
  Field_datetime.


sql/field.h:
  Fixed bug #11385.
  The bug was due to not defined method decimals for the class
  Field_datetime.
mysql-test/t/group_by.test:
  Added  atest case for bug #11385.
mysql-test/r/group_by.result:
  Added  a test case for bug #11385.