io: use case insensitive check for Connection & Upgrade websock headers

When checking the value of the Connection and Upgrade HTTP headers
the websock RFC (6455) requires the comparison to be case insensitive.
The Connection value should be an exact match not a substring.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
