Spec DFC endpoint configuration

It looks like puma finds the file only under `/.well-known/dfc` and not
`/.well-known/dfc/` with a slash in staging environment while it works
here in dev and test.

And in any case, just placing the file in `public/` doesn't produce the
right content type.

spec/requests/well_known_spec.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe "/.well-known/" do
+  describe "dfc/" do
+    it "publishes our endpoints" do
+      get "/.well-known/dfc/"
+
+      expect(response).to have_http_status :ok
+      expect(response.content_type).to eq "text/plain" # Should be JSON!
+      expect(response.body).to include "ReadEnterprise"
+    end
+  end
+end