Using a case insensitive compare on Connection header

@mattmmatt and I were debugging this last night. faye-websocket-ruby wasn't able to connect to nginx because nginx doesn't capitalize "Upgrade" in the connection header.

According to the spec, nginx is probably wrong, but we should still be flexible on the client.

If anyone is interested, here is the relevant line in nginx:
https://github.com/nginx/nginx/blob/master/src/http/ngx_http_header_filter_module.c#L555