PDFBOX-1685: Update the preflight validation following the compatibility requirement exposed in the JIRA issue. RDF:About must be the same in all rdf:description (or empty)

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/branches/1.8@1558521 13f79535-47bb-0310-9956-ffa450edef68