Use only one query to count items in a collection

Hi Guys,

Is there a reason for firing 3 queries to count items in a collection?

The previous version caused 3 queries:
1. `collection.empty?` #=> load an item to ensure there are items in the collection
2. `collection.first ...` #=> load an item again to be sure about we querying mongo
3. `collection.count(true)` #=> actual count command