Concurrent::Map#each_pair should return enumerable when no block given

Author: pitr-ch

lib/concurrent/collection/map/non_concurrent_map_backend.rb

@@ -95,6 +95,7 @@ def clear
       end
 
       def each_pair
+        return enum_for :each_pair unless block_given?
         dupped_backend.each_pair do |k, v|
           yield k, v
         end