even with zero month and day" aka "Date decoding trouble"

Two digit year should be interpreted correctly as year in 20th or 21st
century even with zero month and day. Only exception should be zero date
'00-00-00' or '00-00-00 00:00:00'.


mysql-test/r/type_datetime.result:
  Added test for bug #7297 "Two digit year should be interpreted correctly
  even with zero month and day"
mysql-test/t/type_datetime.test:
  Added test for bug #7297 "Two digit year should be interpreted correctly
  even with zero month and day"
sql/time.cc:
  str_to_TIME(): Two digit year should be interpreted correctly as year
  in 20th or 21st century even with zero month and day. Only exception 
  should be zero date '00-00-00' or '00-00-00 00:00:00'.