Support custom gateway_sessionid

[jerabekjiri]

Add support to the GatewayAuthClient class for custom gateway_sessionid headers. For example aap-dev 2.7 appends port number to the header attribute.

curl -v http://localhost:44927/api/gateway/v1/login/ \
    -H "Referer: http://localhost:44927/login" \
    -H "Cookie: csrftoken=<csrftoken>" \
    -H "X-Csrftoken: <csrftoken> \
    -d "username=<usr>password=<pwd> 2>&1 | grep gateway_sessionid
< set-cookie: gateway_sessionid44927=jwbb69k...