Only restock on_hand inventory units

Example: Spree restocks items once you cancel an order. However it
shouldn't add stock (restock) items from backordered inventory units.
Restock items but dont process backorders for canceled orders

Fixes possible issue where canceling an order with backordered inventory
units would make other orders inventory transition from backordered to
on_hand. e.g.

    - create two separate orders of the same item at the same stock location
    - both orders will have shipment state of backorder
    - cancel one of the orders
    - the second order inventory will transition from backorder to ready

Now the second order inventory remains backorder
Downcase inventory unit state in manifest partial
Only showing unique order records if searching on attributes of associated models like bill_address or line_items

Fixes #3919
Only respond with js if .xhr? is true

Prevents user private data leak for GET requests from other domains
See discussion on rails-core mailing list
https://groups.google.com/forum/#!topic/rubyonrails-core/rwzM8MKJbKU
Remove duplicated product method
Fix frontend features locale_spec

It failed with:

     ActionView::Template::Error:
       undefined method `content_tag' for Spree:Module

Probably due to recent changes in rails i18n api related to the security
patch in 4.0.2, see https://groups.google.com/forum/#!topic/ruby-security-ann/pLrh6DUw998
and https://github.com/rails/rails/commit/78790e4bceedc632cb40f9597792d7e27234138a