IdP-proxy: Avoid unnecessary reauthentication with IDPList.

Don't always reauthenticate with the upstream IdP when IDPList parameter is
specified in the authentication request.

Patch by Benjamin Andersen.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2893 44740490-163a-0410-bde0-09ae8108e29a