Issue: Mailable markdown parses text view as markdown

[clytras]

There is an issue when I try to use a markdown mailable that also is having a text view that will render as plain-text.

class OrderSent extends Mailable
{
    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
        return $this
            ->markdown('emails.orders.order-sent')
            ->text('emails.orders.order-sent-plain');
    }
}

I'm using mail::message component inside both views like @component('mail::message') at the top.

Now when we send the mail, the plain-text there has two issues:

1. The vendor/mail/text view namespace isn't used for plain-text view; instead the html view namespace is used.
2. View actually gets parsed by the markdown parser.

We don't want any of the above when we need to render a plain-text message for emails, it has MIME type of text/plain which any markup is useless and will be displayed to final users.

Screenshot of Mailhog "Plain text" tab showing the vendor/mail/html/layout view:

I did some debugging and I found that Mailable uses buildMarkdownText which it calls renderText only if textView is null

framework/src/Illuminate/Mail/Mailable.php

Lines 345 to 349 in 915d44e

	protected function buildMarkdownText($markdown, $data)
	{
	return $this->textView
	?? $markdown->renderText($this->markdown, $data);
	}

If there is actually a text view, we don't use it and we send the view alias back to the Mailer instead of using renderText and use the proper text view!

If I change this behaviur, and actually use textView if not empty like:

protected function buildMarkdownText($markdown, $data)
{
    return $markdown->renderText($this->textView ?? $markdown, $data);
}

It will actually works and we'll get a clean plain-text message like this:

I consider this to be a bug and of course I can provide a PR if we conclude that this is the intended behaviour.