Make first parameter optional for Schedule method calls

[kylekatarnls]

Allow to call methods such as isOpenOn or isClosedOn without specifying a reference date as first parameter:

$schedule = Schedule::create([...]);

if ($schedule->isOpenOn('Saturday')) {
    // Is typically open on Saturday
}

While it's made available generally by using CarbonImmutable::now() as default reference, it's recommended to use it only for method where the now reference moment is irrelevant. So basically at the moment only isOpenOn and isClosedOn are relevant use cases.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (32212bb) to head (4468091).

:exclamation: Current head 4468091 differs from pull request most recent head 45c0e0b. Consider uploading reports for the commit 45c0e0b to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #69 +/- ## =========================================== Coverage 100.00% 100.00% - Complexity 257 262 +5 =========================================== Files 19 19 Lines 720 730 +10 =========================================== + Hits 720 730 +10 ```

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[codeclimate[bot]]

Code Climate has analyzed commit 45c0e0b4 and detected 4 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Style	3
Clarity	1

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 100.0% (0.0% change).

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