Various fixes from testing of samples (#6)

Minor changes

Author: AdrianSoundy

nanoFramework.Networking.Thread/OpenThread.cs

@@ -187,6 +187,7 @@ public void Start()
             if (!_started)
             {
                 NativeStartThread();
+                _started = true;
             }
             else
             {

nanoFramework.Networking.Thread/OpenThreadDataset.cs

@@ -11,6 +11,7 @@ namespace nanoFramework.Networking.Thread
     /// <summary>
     /// Represents a OpenThread Dataset.
     /// </summary>
+    [Serializable]
     public class OpenThreadDataset
     {
         private UInt16 _channel;

nanoFramework.Networking.Thread/OpenThreadJoinerStartCompleteEventArgs.cs

@@ -19,6 +19,6 @@ internal OpenThreadJoinerStartCompleteEventArgs() : base()
         /// <summary>
         /// Joiner start error or 0 if successful.
         /// </summary>
-        public int error { get => _error; set => value = _error; }
+        public int error { get => _error; set => _error = value; }
     }
 }