Share asks for Basic-Auth while not needed trying to access RSS feeds (thus breaking SSO).

When trying to access RSS feeds through share dashlet link (/share/page/feed/components/dashlets/activities/list?format=atomfeed&mode=user&site=&dateFilter=7&userFilter=all&activityFilter=), an authentication prmpt appear, asking for username and password even if you have a working SSO setup.
In the same session (same SSO crednetials) trying to access the feeds via Alfresco works OK.

Steps to reproduce:

1 - Setup an Alfresco instance
2 - Enable an external authentication in the authentication chain (Jasig CAS server + apache/mod_auth_cas is quite straight forward to setup):
external.authentication.proxyUserName=
external.authentication.proxyHeader=CAS-User
external.authentication.enabled=true
Setup is like this:
Client => Apache Proxy AJP => Share => Alfresco
3 - Configure Share for SSO propagating user through HTTP header.
4 - Modify alfresco-feed end-point to use an SSO aware connector (alfrescoHeader in this case) in share-config-custom.xml:

4 - Test SSO by logging in Share
5 - Click on the RSS link in the My Activities dashlet (or any other RSS link)

Expected behaviour:
Share should trust the HTTP header found in the request and pass it over to Alfresco. Then, Rss feeds should be displayed in the browser using atomfeed format

Actual behaviour:
Share send 401 HTTP status code and the user is prompted for a login and and password. After validating credentials Share passes the request to Alfresco using the SSO HTTP header, without the basic-auth ones.

Additionnal notes:
Same behaviour has been observed using external authentication based on cookies.