title: can't remove user claim in asp Identity
body: <p>i Try to update user claims in my service and at the first i should remove claims like this:  </p>
  
  <pre><code>var userClaimCollection = this.AppUserManager.GetClaims(user.Id);
  var toRemove = new List&lt;Claim&gt;();
  foreach (var claim in userClaimCollection)
  {
        if (claim.Type == group_role)
        {
             toRemove.Add(claim);                    
        }
  }
  foreach(var t in toRemove)
  {
       Claim c = new Claim(group_role, t.Value);
       this.AppUserManager.RemoveClaim(user.Id, c);
  }
  </code></pre>
  
  <p>but on <code>RemoveClaim(user.Id,c)</code> i get <code>Collection was modified; enumeration operation may not execute</code> error.<br>
  what really am i doing wrong?</p>