Fix TimeWithZone#to_s(:rfc822) is deprecated

When running bundle exec rspec, we are getting:

```
DEPRECATION WARNING: TimeWithZone#to_s(:rfc822) is deprecated. Please use TimeWithZone#to_fs(:rfc822) instead. (called from call at ~/upcase/spec/support/host_map.rb:7)
```

This commit resolves the issue by using `TimeWithZone#to_fs(:rfc822)`.

app/views/videos/_video.rss.builder

@@ -5,5 +5,5 @@ xml.item do
  end
  xml.guid video_url(video)
  xml.link video_url(video)
- xml.pubDate video.created_at.to_s(:rfc822)
+ xml.pubDate video.created_at.to_fs(:rfc822)
 end

spec/features/videos_spec.rb

@@ -41,7 +41,7 @@
  .to eq(video_url(published_video))
 
  expect(text_in(item, ".//pubDate"))
- .to eq(published_video.created_at.to_s(:rfc822))
+ .to eq(published_video.created_at.to_fs(:rfc822))
 
  expect(text_in(item, ".//description")).to match(/#{notes}/)
  end