Don't use ProxyFix

[seanh]

ckanserviceprovider always uses the werkzeug proxyfix middleware:

https://github.com/ckan/ckan-service-provider/blob/master/ckanserviceprovider/web.py#L130

(apparently to make it work with gunicorn: https://github.com/ckan/ckan-service-provider/commit/99b675d68980e9aa44b27ad8569724651f4e1fa0)

But the docs say:

Do not use this middleware in non-proxy setups for security reasons.

Should we be using this?