default to default_locale instead of locale

Fixes 8cf88cc75ad0289f39fc2272c372f945b3934e76

Without this change,

    Processing by StoreController#index as HTML
      Parameters: {"locale"=>"es"}
    Completed 500 Internal Server Error in 13ms

    ActiveRecord::StatementInvalid (Could not find table 'cart'):
      app/controllers/concerns/current_cart.rb:7:in `set_cart'

Note: the desired table name is 'carts'.

Failure can be seen here:

http://intertwingly.net/projects/AWDwR4/checkdepot/section-15.2.html