The Item_func_str_to_date class wasn't providing correct integer DATETIME
representation as expected. This led to wrong comparison result and didn't
allowed the STR_TO_DATE function to be used with indexes.
Also, STR_TO_DATE function was inconsisted on throwing warnings/errors.
Fixed now.

val_int and result_as_longlong methods were added to the Item_func_str_to_date
class. 

mysql-test/r/func_time.result:
  Test case result adjusted after fixing bug#56271.
mysql-test/r/parser.result:
  Test case result adjusted after fixing bug#56271.
mysql-test/r/select.result:
  A test case result adjusted after fixing bug#56271.
mysql-test/r/strict.result:
  Test case result adjusted after fixing bug#56271.
mysql-test/r/type_datetime.result:
  Added a test case for the bug#56271.
mysql-test/t/strict.test:
  Test case adjusted after fixing bug#56271.
mysql-test/t/type_datetime.test:
  Added a test case for the bug#56271.
sql/item_timefunc.cc:
  Bug#56271: Wrong comparison result with STR_TO_DATE function
  val_int and result_as_longlong methods were added to the Item_func_str_to_date
  class. 
  Item_func_str_to_date::get_date now throws the ER_WRONG_VALUE_FOR_TYPE warning
  on incorrect value.
sql/item_timefunc.h:
  Bug#56271: Wrong comparison result with STR_TO_DATE function
  val_int and result_as_longlong methods were added to the Item_func_str_to_date
  class.