Fix @klass arel reference to use @table

lib/active_record_extended/query_methods/unionize.rb

@@ -144,7 +144,7 @@ def to_nice_union_sql(color = true)
 
       protected
 
-      def build_unions(arel = @klass.arel_table)
+      def build_unions(arel = @table)
         return unless union_values?
 
         union_nodes      = apply_union_ordering(build_union_nodes!)