IoSession.readBufferSize becomes different from Socket.receiveBufferSize when receiveBufferSize is configured too small.

Calling SocketSessionConfig.setReceiveBufferSize() with very small value (e.g. 512) will cause the specified buffer size to be adjusted to the minimum value supported by operating system (e.g. 1024). IoSession.readBufferSize doesn't check the actual receiveBufferSize value after setting the buffer size, so there's undesired difference between receiveBufferSize and readBufferSize.