title: send a message to a user who has been disconnected for a while in signalR
body: <p>Consider a situation when Implementing a messaging with SignalR, which a users disconnects for a while, and you want to send her again the message she missed. </p>
  
  <p>Here is my simple hub : </p>
  
  <pre><code>[HubName("ContosoChatHub")]
  public class ContosoChatHub : Hub
  {
  
  }
  </code></pre>
  
  <p>And I broadcast the message:</p>
  
  <pre><code>var hubContext = GlobalHost.ConnectionManager.GetHubContext&lt;ContosoChatHub&gt;();
  hubContext.Clients.All.AddCallForAgent(new {  });
  </code></pre>