[5.6] Allow notification routing to depend on notification being sent

[freekmurze]

This PR makes notification routing a bit more flexible by passing the notification to the routeNotificationForXXX

If this would be accepted, this would become possible:

// on a Notifiable

public function routeNotificationForSlack($notification)
{
    if ($notification instanceof MySpecialSnowflakeNotification) {
        return $this->alternative_slack_webhook;
    }
    return $this->slack_webhook;
}

Didn't yet figure out how the new behaviour can be tested properly.

[Dylan-DPC-zz]

Couldn't this be considered breaking because you changed the signature of a public and protected functions that may have some effect?

[taylorotwell]

Merged into 5.6. One question I do have is when would the notification ever be null in RoutesNotifications. I see you made the parameter optional there.

[freekmurze]

The notification will never be null. It was handy to have that parameter as optional while developing the PR. Forgot to make it required when sending the PR.

[Modelizer]

Change log is pointing to 5.5. :/
Also title need to be updated to [5.6]

[aaronhuisinga]

@freekmurze I was just about to open a PR for the same exact thing. Found your Laracasts thread, and found you'd already done it! Thanks!

[meyer59]

Maybe this breaking change should reflect in the upgrade guide to 5.6