[5.4] Ensure Mailable view data is not overridden by order of operations

[jameshulse]

The markdown(), view() and text() methods on the Mailable class currently override the instances viewData field with their own data parameter (which defaults to an empty array).

This means if any of these methods are called after the with() method that the previous view data supplied in the call to with() is lost.

In the following case the view data is empty:

public function build()
{
    $this->view('sample-view')
         ->with([ 'data' => 'example' ])
         ->text('sample-view_plain');
}

This pull request ensures the viewData is always merged with each subsequent call.

Unit tests are provided which exhibit the incorrect behaviour.

[jameshulse]

Thanks @taylorotwell. I can fix the styling issue if you'd like - I'm afraid I didn't run a linter but I'll know for next time :)