[PDFBox-1627] Fix NullPointer on Page thumbnail. Now the PDXObjectImage has a createThumbnailXObject method that uses the same method as the createXObject method in the PDXObject.

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1490022 13f79535-47bb-0310-9956-ffa450edef68