Motivation:
A user might want to handle a certain HTTP upgrade request differently
than what `HttpServerUpgradeHandler` does by default. For example, a
user could let `HttpServerUpgradeHandler` handle HTTP/2 upgrades but
not WebSocket upgrades.
Modifications:
- Added `HttpServerUpgradeHandler.isUpgrade(HttpRequest)` so a user can
tell `HttpServerUpgradeHandler` to pass the request as it is to the
next handler.
Result:
- A user can handle a certain upgrade request specially.
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