User-Agent header is creeping browsers out

[chrstnbwnkl]

By default we're sending a User-Agent header that includes the library name. When using the library in a browser-side project, this sounds the alarms of a couple of modern browsers :D They perform a pre-flight check on the server (since it's CORS). This is a problem at least with the Valhalla instance on OSM, since it doesn't support OPTIONS requests. We should probably remove it since in browser contexts this should be reserved for the browser.