integer constants.

This bug is introduced by the fix for bug#16377. Before the fix the 
Item_func_between::fix_length_and_dec method converted the second and third
arguments to the type of the first argument if they were constant and the first
argument is of the DATE/DATETIME type. That approach worked well for integer
constants and sometimes produced bad result for string constants. The fix for
the bug#16377 wrongly removed that code at all and as a result of this the
comparison of a DATETIME field and an integer constant was carried out in a
wrong way and sometimes led to wrong result sets.

Now the Item_func_between::fix_length_and_dec method converts the second and
third arguments to the type of the first argument if they are constant, the
first argument is of the DATE/DATETIME type and the DATETIME comparator isn't
applicable.


sql/item_cmpfunc.cc:
  Bug#28778: Wrong result of BETWEEN when comparing a DATETIME field with an
  integer constants.
  Now the Item_func_between::fix_length_and_dec method converts the second and
  third arguments to the type of the first argument if they are constant, the
  first argument is of the DATE/DATETIME type and the DATETIME comparator isn't
  applicable.
mysql-test/r/type_datetime.result:
  Added a test case for the bug#28778: Wrong result of BETWEEN when comparing a DATETIME
  field with an integer constants.
mysql-test/t/type_datetime.test:
  Added a test case for the bug#28778: Wrong result of BETWEEN when comparing a DATETIME
  field with an integer constants.