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.