module Shoulda::Matchers::ActionController

def set_session(key)

it { should_not set_session(:user_id) }
it { should set_session(:user_id).to(@user.id) }
it { should set_session(:message) }

Example:

Ensures that a session key was set to the expected value. 
def set_session(key)
  SetSessionMatcher.new(key)
end