adds in geojson download type 

Closes #53 

Happy :poultry_leg: Day!
I'm pretty sure Geoserver doesn't know about application/vnd.geo+json. I just saw your tweet about GeoJSON downloads and thought I'd recommend that you use the official media type. Are you just handing off to a Geoserver request? In that case, there's nothing to be done short of patching Geoserver. But if you're proxying (I can't tell from the code because I don't see the Download class) you could override the Geoserver response and send application/vnd.geo+json to browsers.