allow to detect RDF/OWL xml that starts with xml-comments

```
hets -F https://ontohub.org/repositories/socop/ontologies/5/versions/1
```

should output `RDF document`. 
the xml-version tag should only appear at the beginning. For more entries we simply wait for examples that make changes necessary. (I usually test the files from git://ontohub.org/bioportal.git)