Not adding headers to queueable mailables

[zupolgec]

Fixes #5

There is a bug in Laravel (laravel/framework#17283) that prevents Mailable with callbacks to be serialized for queueing.

So when a Mailable implements ShouldQueue we don't add headers.

Also changed the check from MailableContract to Mailable since the applyDebugHeaders function expects a Mailable (that has a view property).

[jasonvarga]

Doesn't this just remove the debug feature for queued mail?

[zupolgec]

Yes, because this feature is not available on queued Mailables.
This package could provide a modified HeloMailable to extend, but what is the point of debugging locally the content of enqueued Mailables?

[sebwas]

Why has this not been merged yet? @mpociot

[mpociot]

Thank you!