Remove unnecessary duplicate lines

In PR rack#3, headers were converted to lowercase, which resulted in
the same action being performed twice. This PR removes those
duplicate actions.

lib/rack/cache/meta_store.rb

@@ -91,13 +91,12 @@ def store(request, response, entity_store)
       vary = response.vary
       entries =
         read(key).reject do |env, res|
-          (vary == (res['vary'] || res['vary'])) &&
+          (vary == (res['vary'])) &&
             requests_match?(vary, env, stored_env)
         end
 
       headers = persist_response(response)
       headers.delete('age')
-      headers.delete('age')
 
       entries.unshift [stored_env, headers]
       if request.env['rack-cache.use_native_ttl'] && response.fresh?