Purge by key will require an API key to be present

[mmay]

Fastly will begin requiring an API key to be present in purge by key requests starting mid October.

This means that requests to the endpoint - POST https://api.fastly.com/service/:id/purge/:key will require a Fastly-Key or X-Fastly-Key header to be present and contain the correct key.

After browsing source, it looks like if a user is not fully authed, the existing implementation will work, but if the user is fully authed only the cookie is passed, not the API key - see purge_service_by_key and _fetch.

If it helps, you can check out how we handle this in other clients by looking at these PR's in fastly-py and fastly-ruby