[CONFIGURATION-533] DatabaseConfiguration now can deal with CLOB values.

If a property value read from the database is of type java.sql.Clob,
it is automatically converted to a String value.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1466200 13f79535-47bb-0310-9956-ffa450edef68