Change create_join_table migration helper to use table prefixes

Change create_join_table migration helper to use table prefixes like
it's already done when creating HABTM association.

Example:

    class Category < ActiveRecord::Base
      self.table_name = 'catalog_categories'
      has_and_belongs_to_many :products
    end

    class Product < ActiveRecord::Base
      self.table_name = 'catalog_products'
    end

    ActiveRecord::Schema.define do
      create_join_table :catalog_categories, :catalog_products
    end

It created table `catalog_categories_catalog_products` before, but HABTM
association uses `catalog_categories_products` table name:

    `Category.reflections[:products].join_table` #=> "catalog_categories_products"

Now `create_join_table` creates table `catalog_categories_products`,
and it is consistent with what HABTM association does.