Motivation:
At rfc7540 5.5, it said that it's not permitted to send extension
frame in the middle of header block and need be treated as
protocol error
Modifications:
When received a extension frame, in netty it's called unknown frame,
will verify that is there an headersContinuation exists
Result:
When received a extension frame in the middle of header block,
will throw connection error and closed the connection
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