fluid.fetchResources stalls if defaultLocale and locale are identical, and forceCache option is supplied

Reported in #fluid-work at https://botbot.me/freenode/fluid-work/2015-05-06/?msg=38471885&page=1

The issue is that our "cache" only caches successes, not failures - therefore a request which fails, but which is requested from the cache, will never have its callback notified. Setting defaultLocale and locale to the same value can expose this bug, since it enters two requests into the collection which are for the same URL, but which is not present.