将DeviceManager中的_imageBuffer、_imageDataAccumulator和_multipartBoundary字段设为只读，解决IDE0044警告

Windows/CS/Framework4.0/Toprie/Toprie/DeviceManager.cs

@@ -214,11 +214,11 @@ namespace JoyD.Windows.CS.Toprie
         // 停止请求事件
         private ManualResetEvent _stopRequested = new ManualResetEvent(false);
         // 缓冲区
-        private byte[] _imageBuffer = new byte[4096];
+        private readonly byte[] _imageBuffer = new byte[4096];
         // 图像数据累积缓冲区
-        private byte[] _imageDataAccumulator = new byte[0];
+        private readonly byte[] _imageDataAccumulator = new byte[0];
         // 多部分请求边界
-        private string _multipartBoundary = string.Empty;
+        private readonly string _multipartBoundary = string.Empty;
         // 锁对象
         private readonly object _lockObject = new object();
         // 是否启用自动重连