Extraneous error message when using RDF files

When reading or writing RDF files, the following error is logged:

```
[main] ERROR org.apache.jena.util.JenaXMLInput - Problem setting StAX property - name: "http://javax.xml.xmlconstants/property/accessExternalDTD" - value: "" - error: Unrecognized property 'http://javax.xml.xmlconstants/property/accessExternalDTD'
```

Note: This issue is split out from issue #88 