After this change the DefaultHandshakeHandler delegates to a
server-specific RequestUpgradeStrategy to update the HTTP response for
the handshake request and to begin the WebSocket interaction.
The DefaultHandshakeHandler however still retains the initial
validation of the WebSocket handshake including negotation of origin,
sub-protocol, etc. This allows sub-classes to override various
aspects of the negotiation independant of the WebSocket engine.
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