[8.x] Account for a numerical array of views in Mailable::renderForAssertions()

[paulredmond]

This attempts to account for a numerical array returned from Mailable::buildView() within the new renderForAssertions() method introduced in afb858a. The update isn't elegant, but buildView can return a string so this checks if the $view is an array and if the numeric index is set.

I did not see any existing tests and realized it'd take more work to add template fixtures, container support, etc. to existing tests so please review carefully in case I missed something.

Steps to reproduce

// in a mailable
public function build()
{
    return $this->view('emails.my-email')
                ->text('emails.my-email-plain');
}

In the Mailable::renderForAssertions() method:

$html = Container::getInstance()->make('mailer')->render(
    $view = $this->buildView(), $this->buildViewData()
);

dd($view);
/*
array:2 [
  0 => "emails.my-email"
  1 => "emails.my-email-plain"
]
*/

The current logic (before this fix) works as expected when you use markdown or HTML in which Mailable::buildView() returns an associative array:

    return $this->html('Hello World')
                ->text('emails.my-email-plain');

/*
array:2 [
  "html" => Illuminate\Support\HtmlString^ {#114
    #html: "Hello World"
  }
  "text" => "emails.my-email-plain"
]
*/

Similar logic can be found in Mailer::parseView.