/// <summary>
/// Asynchronously sends the specified message.
/// </summary>
/// <param name="message">The message to send.</param>
/// <param name="cancellationToken">A token to monitor for cancellation requests. The default value is <see cref="System.Threading.CancellationToken.None"/>.</param>
/// <returns>The message.</returns>
public async Task SendAsync(Message message, CancellationToken cancellationToken = default(CancellationToken))
{
if (message == null) throw new ArgumentNullException("message");
if (_isDisposed) throw new ObjectDisposedException("Client");
if (ConnectionState != ClientConnectionState.Established) throw new InvalidOperationException("Client must be connected.");
var json = message.ToJson();
var response = await Http.PostJsonAsync("https://slack.com/api/chat.postMessage", json, cancellationToken, "token", _token);
if (response["ok"] != "true")
throw SlackException.FromStatusCode(response["error"]);
}