mod_websocket: Fire pre-session-close event (fixes #1800) This event was added in a7c183bb4e64 and is required to make mod_smacks know that a session was intentionally closed and shouldn't be hibernated (see fcea4d9e7502). Because this was missing from mod_websocket's session.close(), mod_smacks would always attempt to hibernate websocket sessions even if they closed cleanly. That mod_websocket has its own copy of session.close() is something to fix another day (probably not in the stable branch). So for now this commit makes the minimal change to get things working again. Thanks to Damian and the Jitsi team for reporting.

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