title: undefined method `session' for nil:NilClass in RSpec Rails
body: <p>There is the following helper for my specs in Rails:</p>
  
  <pre><code>include ApplicationHelper
  
  def sign_in(user)
    session[:access_token] = user.access_token
  end
  
  def signed_in?
    current_user.present?
  end
  
  def current_user
    User.find_by(access_token: session[:access_token])
  end
  </code></pre>
  
  <p>But I've got "undefined method `session' for nil:NilClass". How can I fix it? Thanks in advance. </p>