This allows for web servers to be "hijacked" and used as Web Socket servers
(or other). You simply listen for requests as normal, but check if
req.upgrade === true
If so, this will be the last request of the connection. It's your job now to
hijack req.connection and start reading from it. req.upgradeHead is a buffer
containing the first part of the new protocol communication (in the case it
arrived on the same packet).
This needs tests and documentation. API subject to change.
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