Add assertNoRedirect method.

Might be a useful addition to the test case.

I often check for redirects with

    $this->assertResponseOk();
    $this->assertRedirect($url);

But when some response should specifically not redirect, this is more difficult to assert.
This PR makes this possible quite easily:

    $this->assertResponseOk();
    $this->assertNoRedirect();


Opening but not closing a buffer usually results in a risky test which is effectively skipped by phpunit.