saml: Fix SAML 2.0 SP logout with PostgreSQL.

The PostgreSQL database server returns all column names in lowercase,
but we are using mixed case column names. This patch changes the query
to always request lowercase column names.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3207 44740490-163a-0410-bde0-09ae8108e29a