[Modify] Add a field

It can be used as the empty payload data.

websocket-sharp/PayloadData.cs

@@ -45,6 +45,11 @@ namespace WebSocketSharp
 
     #region Public Fields
 
+    /// <summary>
+    /// Represents the empty payload data.
+    /// </summary>
+    public static readonly PayloadData Empty = new PayloadData ();
+
     /// <summary>
     /// Represents the allowable max length.
     /// </summary>