Use a public API to set the table alias

Since they dropped the table_alias writer on the Arel::Table, we're
no longer modifying it so we can drop the dup.

Author: jrafanie

lib/active_record/virtual_attributes/virtual_delegates.rb

@@ -272,10 +272,8 @@ def self.select_from_alias_table(to_klass, src_relation)
         to_table = to_klass.arel_table
         # if a self join, alias the second table to a different name
         if to_table.name == src_relation.name
-          # use a dup to not modify the primary table in the model
-          to_table = to_table.dup
           # use a table alias to not conflict with table name in the primary query
-          to_table.instance_variable_set(:@table_alias, "#{to_table.name}_sub")
+          to_table = to_table.alias("#{to_table.name}_sub")
         end
         to_table
       end