Dispose objects in PayloadWriter.

These objects implement IDisposable and trigger (false positive)
analysis warnings about undisposed objects.

PayloadWriter should not be made IDisposable because it could be
rewritten to use ArrayPool (and manage its own memory); clients don't
need to worry about this implementation detail.

src/MySqlConnector/Serialization/PayloadWriter.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.IO;
 using System.Text;
 
@@ -53,7 +53,9 @@ public void WriteNullTerminatedString(string value)
 		public byte[] ToBytes()
 		{
 			m_writer.Flush();
-			return m_stream.ToArray();
+			using (m_writer)
+			using (m_stream)
+				return m_stream.ToArray();
 		}
 
 		readonly MemoryStream m_stream;