When doing indexed search the server constructs a key image for 
faster comparison to the stored keys. While doing that it must not
perform (and stop if they fail) the additional date checks that can 
be turned on by the SQL mode because there already may be values in 
the table that don't comply with the error checks.
Fixed by ignoring these SQL mode bits while making the key image.


mysql-test/r/type_date.result:
  Bug #28687: test case
mysql-test/t/type_date.test:
  Bug #28687: test case
sql/item.cc:
  Bug #28687: no invalid date warnings