JSR356Endpoint discards header values

[Artur-]

When using Firefox, Glassfish 4 and JSR-356, websockets do not work. The upgrade process is ok but after that you get
Status code higher or equal than 400. Unable to deliver the websocket messages to installed component. Status 501 Message OK

The reason is that Firefox sends a
Connection: keep-alive, Upgrade
header in the upgrade request and https://github.com/Atmosphere/atmosphere/blob/master/modules/cpr/src/main/java/org/atmosphere/container/JSR356Endpoint.java#L126
discards everything but the first value, which happens to be "keep-alive" in this case.

Unsurprisingly, a later verification that a "Connection: upgrade" header exists fails because of this.