This commit restores the ability to assign cookies for testing via
@request.env['HTTP_COOKIE'] and @request.cookies, e.g:

    @request.env['HTTP_COOKIE'] = 'user_name=david'
    get :index
    assert_equal 'david', cookies[:user_name]

and

    @request.cookies[:user_name] = 'david'
    get :index
    assert_equal 'david', cookies[:user_name]

Assigning via cookies[] is the preferred method and will take precedence
over the other two methods. This is so that cookies set in controller
actions have precedence and are carried over between calls to get, post, etc.