Lowercase header key in test

Rack 3 now lowercases all header keys [1].

Therefore we need to lowercase the keys in test expectations.

1: https://rubydoc.info/github/rack/rack/Rack/Headers

spec/integration/fetching_content_item_spec.rb

@@ -87,7 +87,7 @@
     end
 
     it "sets a access-control-allow-origin header to allow cross origin requests" do
-      expect(response.headers.to_h).to include({ "Access-Control-Allow-Origin" => "*" })
+      expect(response.headers.to_h).to include({ "access-control-allow-origin" => "*" })
     end
 
     context "when the user is not authenticated" do