The fix for bug 31887 was incomplete : it assumes that all the 
field types returned by the IS_NUM macro are descendants of 
Item_num and tries to zero-fill the values before doing constant
substitution with such fields when they are compared to constant string
values.
The only exception to this is Field_timestamp : it's in the IS_NUM
macro, but is not a descendant of Field_num.
Fixed by excluding timestamp fields (Field_timestamp) when zero-filling
when converting the constant to compare with to a string.
Note that this will not exclude the timestamp columns from const 
propagation.

mysql-test/r/compare.result:
  Bug #39353: test case
mysql-test/t/compare.test:
  Bug #39353: test case
sql/item.cc:
  Bug #39353: don't zero-fill timestamp fields when const propagating
  to a string : they'll be converted to a string in a date/time format
  and not as an integer.