…support CORS

GPX file used always the service-proxy even if the server supported CORS. Also
on the service-proxy we could see that some requests were made over service-proxy
for internal server like public.geo.admin.ch. This was due to the fact that if
for any reason the initial request failed (network failure) we fallback to the
service proxy. Now in that case we don't fallback anymore.

NOTE unfortunately there is no way for a web application to check for CORS support
see https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS/Errors

PB-977: Don't use service-proxy for internal domain and for GPX that support CORS - #patch