Query for negative associations as subquery

When a collection association (has_many, etc) is searched for negative
conditions (NOT...), a JOIN will still include other rows that match.

The implied meaning is that it should only select where *none* of the
associations match, but the actual result still selects records where
*any* of the joined associations match.

This implementation removes joins that were added while building the
conditions and moves them into a subquery if needed.

lib/ransack/adapters/active_record/ransack/context.rb

@@ -27,6 +27,8 @@ module Ransack
       @join_dependency = join_dependency(@object)
       @join_type = options[:join_type] || Polyamorous::OuterJoin
       @search_key = options[:search_key] || Ransack.options[:search_key]
+      @associations_pot = {}
+      @lock_associations = []
 
       if ::ActiveRecord::VERSION::STRING >= Constants::RAILS_4_1
         @base = @join_dependency.join_root