[8.x] Use "Conditionable" in existing classes that implement when()

[inxilpro]

Right now, BuildsQueries, Mailable, MailMessage, and Stringable all implement their own version of when() that is exactly the same as the implementation in the new Conditionable trait. This PR just swaps them out for the new trait instead.

(This means that Mailable gets an unless() method as well. All other classes had both a when() and unless() method already.)

It's also worth noting that unless() was implemented in two different ways in the framework. One implementation was a mirror of the when() implementation, while another approach was to call $this->when(! $value, ...). It turns out that the latter solution (calling when()) is incorrect, because it ends up passing the result of ! $value to the callback, rather than the actual value. This PR fixes that issue in the Conditionable trait, thus standardizing it across all the classes that use it.

[bonroyage]

@inxilpro you can also add it to Illuminate\Support\Traits\EnumeratesValues while still maintaining the higher order functionality.

    use Conditionable {
        Conditionable::when as conditionableWhen;
    }

    public function when($value, callable $callback = null, callable $default = null)
    {
        if (! $callback) {
            return new HigherOrderWhenProxy($this, $value);
        }

        return $this->conditionableWhen($value, $callback, $default);
    }

[taylorotwell]

Is there any change in behavior at all? Also, please keep traits in alphabetical order like the rest of the framework.

[inxilpro]

Is there any change in behavior at all?

The only change in behavior is technically a bug fix:

Str::of('foo')->unless(null, fn($string, $value) => assert(is_null($value)));

As of Laravel 8.5.19, this will trigger a warning, because the Stringable implementation of unless() will pass a true to the callback rather than the original null. After this change, the above will work.

I'll add a test to demonstrate.

[inxilpro]

@inxilpro you can also add it to Illuminate\Support\Traits\EnumeratesValues

I thought about that, but honestly, it feels much clearer to just leave it as-is.