title: Facebook login returns error message if domain accessed without www prefix
body: <p>I have OmniAuthable Facebook login on my website. Users login with:</p>
  
  <pre><code>link_to user_omniauth_authorize_path(:facebook)
  </code></pre>
  
  <p>It is defined in app config at developers.facebook.com.</p>
  
  <p>If I access the website through www.example.com, everything works ok and I can login.</p>
  
  <p>If I access it without the "www" subdomain (i.e. example.com), I get the following response:</p>
  
  <pre><code>{
     "error": {
        "message": "Invalid redirect_uri: Given URL is not allowed by the Application configuration.",
        "type": "OAuthException"
     }
  }
  </code></pre>
  
  <p>Does anybody know how I can fix this?</p>