How to test file download using maatwebsite/excel

I am using maatwebsite/excel to return an excel file. This works fine but I want to be able to test this using phpunit.

I have a controller method which returns the excel file.

public function usersAll(){

        $users = User::all();

         return Excel::create('users', function($excel) use ($users) {

           $excel->sheet('Users', function($sheet) use ($users) {

                $sheet->loadView('csv.users')->with('users', $users);

            });

        })->export('xls');
    }       

I want to use phpunit to hit a route and assert that the response is ok. Something like this:

  /** @test */
    public function test_csv()
    {
        $response = $this->call('GET','/csv/users/all');

        $this->assertResponseOk();    
    }

What is the best way to test this, I receive error that headers are already sent. I have been digging around google for a solution but cant find anything to solve this.

Does anybody else test for this using this package? I would love to know the best way forward.

Thanks.