…used together with Http2MultiplexHandler (netty#9495)

    Motivation:

    In the release (4.1.37) we introduced Http2MultiplexHandler as a
    replacement of Http2MultiplexCodec. This did split the frame parsing from
    the multiplexing to allow a more flexible way to handle frames and to make
    the code cleaner. Unfortunally we did miss to special handle this in
    Http2ClientUpgradeCodec and so did not correctly add Http2MultiplexHandler
    to the pipeline before calling Http2FrameCodec.onHttpClientUpgrade(...).
    This did lead to the situation that we did not correctly receive the event
    on the Http2MultiplexHandler and so did not correctly created the
    Http2StreamChannel for the upgrade stream. Because of this we ended up
    with an NPE if a frame was dispatched to the upgrade stream later on.

    Modifications:

    - Correctly add Http2MultiplexHandler to the pipeline before calling Http2FrameCodec.onHttpClientUpgrade(...)

    Result:

    Fixes netty#9495.
    Added Unit Test