Allow an array for URLs to be consistent with assertRedirect().

post(), get() etc should also allow an array, just as in 2.x.
This also unifies it regarding assertRedirect() and in most cases does not require an additional "Router" use  statement then in the application test files.

    $url = ['controller' => 'Posts', 'action' => 'add'];
    $this->post($url, $data);
    $anotherUrl = ['controller' => 'Posts', 'action' => 'index'];
    $this->assertRedirect($anotherUrl);