fix response header

[ramonawalls]

From Simon Cox:

BTW – http://purl.obolibrary.org/obo/bco.owl does not really follow the rules – in the http response header it gives Content-Type: text/plain regardless of what’s in the request Accept: header, which means it doesn’t play nice with RDF tools. I have taken a local cache, and made some tweaks but still completely failed to load it in TopBraid L