Improve stuffs about assertion call and assertions

Created by: peter279k

Changed log

• Using the assertNull to assert expected is null.
• PHPUnit has two ways to make assertion call. One is $this and another one is self::. It seems that using self:: times is greater than using $this. To be consistency, using the self to make assertion call on this repository.
• Using the assertSame to replace assertEquals and it can let all assertions make equals checking strictly.