Motivation:
The CorsHandler currently closes the channel when it responds to a preflight (OPTIONS)
request or in the event of a short circuit due to failed validation.
Especially in an environment where there's a proxy in front of the service this causes
unnecessary connection churn.
Modifications:
CorsHandler now uses HttpUtil to determine if the connection should be closed
after responding and to set the Connection header on the response.
Result:
Channel will stay open when the CorsHandler responds unless the client specifies otherwise
or the protocol version is HTTP/1.0
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