Fix exception in DisposeAsync. Fixes #305

If the connection is not in a known state (Connected or Failed), we'll just shut down the socket, which may be logged on the server as an aborted connection. This should prevent failure to shut down the session when an exception is thrown,  leaking connections to the server.

src/MySqlConnector/Serialization/MySqlSession.cs

@@ -153,22 +153,28 @@ public async Task DisposeAsync(IOBehavior ioBehavior, CancellationToken cancella
 			if (m_payloadHandler != null)
 			{
 				// attempt to gracefully close the connection, ignoring any errors (it may have been closed already by the server, etc.)
+				State state;
 				lock (m_lock)
 				{
-					VerifyState(State.Connected, State.Failed);
-					m_state = State.Closing;
+					if (m_state == State.Connected || m_state == State.Failed)
+						m_state = State.Closing;
+					state = m_state;
 				}
-				try
-				{
-					m_payloadHandler.StartNewConversation();
-					await m_payloadHandler.WritePayloadAsync(QuitPayload.Create(), ioBehavior).ConfigureAwait(false);
-					await m_payloadHandler.ReadPayloadAsync(m_payloadCache, ProtocolErrorBehavior.Ignore, ioBehavior).ConfigureAwait(false);
-				}
-				catch (IOException)
-				{
-				}
-				catch (SocketException)
+
+				if (state == State.Closing)
 				{
+					try
+					{
+						m_payloadHandler.StartNewConversation();
+						await m_payloadHandler.WritePayloadAsync(QuitPayload.Create(), ioBehavior).ConfigureAwait(false);
+						await m_payloadHandler.ReadPayloadAsync(m_payloadCache, ProtocolErrorBehavior.Ignore, ioBehavior).ConfigureAwait(false);
+					}
+					catch (IOException)
+					{
+					}
+					catch (SocketException)
+					{
+					}
 				}
 			}
 			ShutdownSocket();
@@ -195,11 +201,7 @@ public async Task ConnectAsync(ConnectionSettings cs, IOBehavior ioBehavior, Can
 				throw new MySqlException("Unable to connect to any of the specified MySQL hosts.");
 			}
 
-#if NETSTANDARD2_0
- 			var byteHandler = new StreamByteHandler(m_networkStream);
-#else
 			var byteHandler = new SocketByteHandler(m_socket);
-#endif
 			m_payloadHandler = new StandardPayloadHandler(byteHandler);
 
 			var payload = await ReceiveAsync(ioBehavior, cancellationToken).ConfigureAwait(false);