[5.5] Allow HtmlString as line in notifications MailMessage

[webkonstantin]

Allows for

public function toMail($notifiable)
{
    $line = new HtmlString('Hello <strong>world</strong>');
    
    return (new MailMessage)->with($line);
}

[sisve]

Perhaps allow the Htmlable interface instead of the HtmlString implementation?

[webkonstantin]

@sisve sure. fixed, thanks

[Miguel-Serejo]

Should have changed the title to reflect the change, now the change list in the tag is wrong.

https://github.com/laravel/framework/releases/tag/v5.5.27