mongoid/mongoid — pull request #3847 by cheredon (2014-10-31 09:09:36+00:00)
Title: Pluck duplicates first 101 records for query cache

Hi, I use QueryCache in project. When I call .pluck(:id) at first time result duplicates first 101 ids.
```
2.0.0-p576 :029 > Mongoid::QueryCache.clear_cache
=> nil
2.0.0-p576 :030 > Mongoid::QueryCache.cache {Unit.where(source: s).pluck(:id).count}
=> 677
2.0.0-p576 :031 > Mongoid::QueryCache.cache {Unit.where(source: s).pluck(:id).count}
=> 576
```

In fact there are 576 records in database.