Fix AUTH packet fixed header

[lizhanhui]

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Analyze AUTH packet, the reserved part of the fixed header:

According to MQTT protocol 5.0 AUTH packet, fixed header, byte 1, reserved part shall be all 0:

1.2.5 client, when talking to netty-mqtt-codec server, an error reports:

io.netty.handler.codec.DecoderException: Illegal BIT 0 in fixed header of AUTH message, must be 0, found 1

This pull request fixes:

#980

#981