…today's date and a format is specified

The issue was that the time part was bleeding through from the current timestamp in `DateTime::createFromFormat`.
Test case:
* Validation rule: 'birth_date' => ['required', 'date_format:Y-m-d', 'before_or_equal:today']
* '2017-08-27' was parsed as '2017-08-27 22:41:37'.
* 'today' was parsed as '2017-08-27 00:00:00'.
* Validation failed.

This shows that the fix in 1b651e6 is not going to break applications, people can still use the time part.
This should address the comment of @taylorotwell at laravel#20789 .

…at-5.5

Also added a number of test cases.
The issue was that `===` always returns false for the two separate DateTime instances returned by `getDateTimeWithOptionalFormat`.