Fix GridFS initial query for documents:

- Negative limit values are invalid on 3.2 and higher.
- Use a projection to only deserialize the _id field.
- Don't convert to array - only need the first document.

Author: durran

lib/mongo/grid/fs_bucket.rb

@@ -438,7 +438,7 @@ def files_name
       end
 
       def ensure_indexes!
-        if files_collection.find({}, limit: -1).to_a.empty?
+        if files_collection.find({}, limit: 1, projection: { _id: 1 }).first.nil?
           chunks_collection.indexes.create_one(FSBucket::CHUNKS_INDEX, :unique => true)
           files_collection.indexes.create_one(FSBucket::FILES_INDEX)
         end