Update header detection for BIG-IP load balancer

With the BIG-IP load balancers, the OneConnect feature override the `connection` header and copy its content into the `x-cnection` header ([Documentation](http://support.f5.com/kb/en-us/solutions/public/6000/900/sol6997.html)).

Behind this load balancer SockJS fail to upgrade to WebSocket because it only check the `connection` header for "Upgrade". We should also check the `x-cnection` header.

This patch has no incidences on servers that do not use BIG-IP (the `x-cnection` header does not exists and we fallback to the `connection` header).