OWLOntologyWriter could support more mime types

Currently the OWLOntologyWriter miss the following mime types definitions:

• text/n3
• text/plain (interpreted as N3 notation)
• text/turtle

The reason is to align the mime type definitions with the Jena writer, and to be more flexible (sometimes a Turtle format could be asked with the text/turtle mimetype instead of the mapped application/turtle, for example)