title: How to get current UserId in Identity 3.0? User.GetUserId returns null
body: <p>I need to get ID of user that made some request.
  In previous versions of Identity I could do it like this:</p>
  
  <pre><code>User.Identity.GetUserId();
  </code></pre>
  
  <p>But it seems it isn't available anymore.</p>
  
  <p>There is also something like:</p>
  
  <pre><code>User.GetUserId();
  </code></pre>
  
  <p>But it always returns null, even if User.Identity.IsAuthenticated in true and User.Identity.Name is set correctly.</p>
  
  <p>What should I use to do it?</p>
  
  <p>EDIT:
  My authenticaton logic is based on <a href="https://github.com/MyPCIsBetter/ASP.net-core-rc1-default-Visual-Studio-2015-template/tree/master/src/ASPnet5webtemplate" rel="noreferrer">[default Visual Studio 2015 template]</a>, I hadn't changed much so far in identity, so if you want to check how is it done, you can simply see it on github link I pasted.</p>