CLC-3187: LiveUpdates: Do warmup in a forced-cache-write mode (Part 3)

Fixed test failure in UserCacheWarmer
https://jira.media.berkeley.edu/jira/browse/CLC-3187

See duplicate: CLC-3211: Test failure in UserCacheWarmer
https://jira.media.berkeley.edu/jira/browse/CLC-3211

The user_cache_warmer test broke because get_feed was getting called twice instead of once.  This was because the UserCacheWarmer was changed to call model.get_feed_as_json, in addition to get_feed, and the previous stub of get_feed was getting called twice instead of once.  Revised this test to make it clear that both get_feed and get_feed_as_json are each called only once by the UserCacheWarmer.