Fix for case-insensitive Upgrade header match.
Minimum JDK requirement is changed to JDK 7.
Conforms to RFC6455 specification:
https://datatracker.ietf.org/doc/html/rfc6455#page-56
If the response lacks a |Connection| header field or the
|Connection| header field doesn't contain a token that is an
ASCII case-insensitive match for the value "Upgrade", the client
MUST _Fail the WebSocket Connection_.