Added exec_query to read only method list. Looks like only place used in...

In ActiveRecord 3.2 :pluck uses exec_query to query database. currently exec_query is proxyed using master always. However pluck is a read only call and only place in ActiveRecord 3.2 exec_query is used. Adding exec_query to the list of read only methods so that .pluck(:id) etc will use the configured pool instead of always the master pool.