* Fix #2619 - When redis feed is empty, fall back to database

* Use redis value to return feed from database only while RegenerationWorker
hasn't finished running

* Fix specs

* Replace usage of reject!

This adds a test for the `Feed#get` method. While the data
transformations in `Feed#get` may seem redundant, they are important to
maintain the order from Redis. The tests I wrote will fail if someone
tries to refactor away this "redundancy" (as I tried to do in the first
iteration of this change).