TypeError: ActiveSupport::TimeWithZone can't be coerced into Float

Make CacheItem's expires_at type comply with ActiveSupporrt::Cache::Store::Entry's type.

ActiveSupport::Cache::Store's fetch method attempts the following to determine if a cached entry is past its expiration -- normally using an ActiveSupport::Cache::Store::Entry whose expires_at attribute returns the number of seconds since the epoch:

```if race_ttl and Time.now.to_f - entry.expires_at <= race_ttl```

The ActiveSupport::Cache::ActiveRecordStore::CacheItem class' expires_at reader should conform to this expectation as to avoid failed coercions on cache timeout.