public function testOnly()
 {
     $data = new Collection(['first' => 'Taylor', 'last' => 'Otwell', 'email' => '*****@*****.**']);
     $this->assertEquals(['first' => 'Taylor'], $data->only(['first', 'missing'])->all());
     $this->assertEquals(['first' => 'Taylor'], $data->only('first', 'missing')->all());
     $this->assertEquals(['first' => 'Taylor', 'email' => '*****@*****.**'], $data->only(['first', 'email'])->all());
     $this->assertEquals(['first' => 'Taylor', 'email' => '*****@*****.**'], $data->only('first', 'email')->all());
 }