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.
Merge pull request #784 from ontohub/783-cooperate_with_owl_api

783 cooperate with owl api