Force download file on OWL API HTTP accept headers

Temporarily fixes #783.

The OWL tools somehow make two different HTTP requests.
One type has the accept header
"application/rdf+xml, application/xml; q=0.5, text/xml; q=0.3, */*; q=0.2"
while the other type has the accept header
"text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2".
However, they both are used to get the exact same .owl file.

This commit is about to be reverted as soon as the OWL tools send
consistent request headers.