[MODEL] Refactored the code in __find_in_batches to use Enumerable#each_slice

Closes elastic#378

Author: lompy

elasticsearch-model/lib/elasticsearch/model/adapters/mongoid.rb

@@ -64,18 +64,8 @@ module Importing
           #
           def __find_in_batches(options={}, &block)
             options[:batch_size] ||= 1_000
-            items = []
-
-            all.no_timeout.each do |item|
-              items << item
-
-              if items.length % options[:batch_size] == 0
-                yield items
-                items = []
-              end
-            end
-
-            unless items.empty?
+  
+            all.no_timeout.each_slice(options[:batch_size]) do |items|
               yield items
             end
           end