update .net core startup

src/Emby.Server/IO/MemoryStreamFactory.cs

@@ -0,0 +1,33 @@
+using System;
+using System.IO;
+using MediaBrowser.Model.IO;
+
+namespace Emby.Server.IO
+{
+    public class MemoryStreamFactory : IMemoryStreamFactory
+    {
+        public MemoryStream CreateNew()
+        {
+            return new MemoryStream();
+        }
+
+        public MemoryStream CreateNew(int capacity)
+        {
+            return new MemoryStream(capacity);
+        }
+
+        public MemoryStream CreateNew(byte[] buffer)
+        {
+            return new MemoryStream(buffer);
+        }
+
+        public bool TryGetBuffer(MemoryStream stream, out byte[] buffer)
+        {
+            ArraySegment<byte> arrayBuffer;
+            stream.TryGetBuffer(out arrayBuffer);
+
+            buffer = arrayBuffer.Array;
+            return true;
+        }
+    }
+}