title: ReturnUrl is null in ASP.NET Core login
body: <p>I have ASP.NET Core application with individual accounts; very similar to what gets generated by VS2017. For testing purposes I put <code>[Authorize]</code> attribute on <code>About()</code> action in Home controller. I am redirected to Login page as expected, and I see that URL is <code>http://localhost:5000/Account/Login?ReturnUrl=%2FHome%2FAbout</code> - also as expected. However, in the POST Login method ReturnUrl is null. I have Login method in Account Controller:</p>
  
  <pre><code>[HttpPost]
  [AllowAnonymous]
  [ValidateAntiForgeryToken]
  public async Task&lt;IActionResult&gt; Login(LoginInputModel model) {
  ...
  }
  </code></pre>
  
  <p>I also tried <code>ReturnUrl</code> as parameter explicitly, with or without <code>[FromQuery]</code>. In all permutations it is null.</p>