#put method does not respect sessions in functional tests

In my functional tests I'm testing two actions in two different controllers. The first action "save" in the SettingsController is tested like this:

login_as :quentin
put :save, {}, :settings => [{:id => settings(:site_name).id, :value => "Hello world"}]
assert_redirected_to :controller => "settings", :action => "index"

The #login_as helper method looks like this:

def login_as(user)
  @request.session[:user_id] = user ? users(user).id : nil
end

It assigns a session variable to the request. My other action "update" in the PagesController is also tested like above, using the #put method that makes a new PUT request. But the two tests fails and the failure message is similar to this:

expected a redirect to <{"action"=>"index", "controller"=>"settings"}>, found one to <{"action"=>"ne
w", "controller"=>"sessions"}>, a difference of <{"action"=>"new", "controller"=>"sessions"}>

What this actually means is, that there is no "user_id" session variable which must mean that #put doesn't respect sessions. Can anyone confirm this?