Use .flat_map{} instead of .map{}.flatten

In this particular case, they are semantically equivalent,
but flat_map is shorter and faster.

lib/chewy/type/import.rb

@@ -79,7 +79,7 @@ def bulk_body(action_objects, indexed_objects = nil)
           action_objects.flat_map do |action, objects|
             method = "#{action}_bulk_entry"
             crutches = Chewy::Type::Crutch::Crutches.new self, objects
-            objects.map { |object| send(method, object, indexed_objects, crutches) }.flatten
+            objects.flat_map { |object| send(method, object, indexed_objects, crutches) }
           end
         end