[5.7] Mailable render() respects Mailable@locale

[derekmd]

laravel/ideas#1318

When testing mailables in the browser or calling Mailable@render() in production (e.g., to store the HTML string in the database), this will render in Mailable@locale when it is filled.

Testing example:

$mail = new App\Mail\OrderConfirmation(App\Order::first());
$translatedHtml = $mail->locale('es')->render();

Production example:

$mail = new OrderConfirmation($order);
Mail::to($user)->queue($mail);

// this would be in $user->preferredLocale()
$translatedHtml = $mail->render();