Allow testing of a specific status code.

Asserts often times should be as specific as possible, if there was a specific code returned, for example, it should rather check on that one then instead of comparing to a wider range. Similar to assertSame() vs assertEquals(). E.g. when using 301/302.

What do you think of allowing

    $this->assertResponse($specificCode);

here?

If accepted, I will add a few tests.