Count * after more than one field were selected

I am using ActiveRecord 4.0 and noticed that some of my queries failed due to problems in calling COUNT function:

```
2.0.0p0 :001 > Organization.select("id, name").count
   (0.5ms)  SELECT COUNT(id, name) FROM "organizations"
PG::UndefinedFunction: ERROR:  function count(integer, character varying) does not exist
LINE 1: SELECT COUNT(id, name) FROM "organizations"
```

I've created patch to use existing correct implementation of method `select_for_count` from parent module `RelationExtensions` and added few lines to make it work similar to its fresh analogue in rails
https://github.com/rails/rails/commit/648afea0e4addcaaf6da31cba78b94d7681706a4