fix(cache): treat caches as private

Not making this breaking because tbh it was kind of unexpected. Shared
caches are much less aggressive about caching, specially when it comes
to authenticated requests.

In the future, make-fetch-happen should allow this to be configurable,
but for now, caches should be considered private for server-side
purposes.

index.js

@@ -139,7 +139,9 @@ function makePolicy (req, res) {
     status: res.status,
     headers: adaptHeaders(res.headers)
   }
-  return new CachePolicy(_req, _res)
+  return new CachePolicy(_req, _res, {
+    shared: false
+  })
 }
 
 // https://tools.ietf.org/html/rfc7234#section-4.2