[api] Fix 1.8 syntax errors in ProductProperties, ReturnAuthorizations and Taxonomies controllers
Introduced datepicker_field_value helper method for datepicker fields
Allows old orders to get promo adjustments

UNACTIVATABLE_ORDER_STATES already covers scenarios where a promo should
not be applied to an order

Fixes #2388

Fixes #2395
Freeze to rspec-rails 2.12.0 due to this commit:

https://github.com/rspec/rspec-rails/commit/09793f885637ee4d6f5cc2c10b4f60b078acb9b5#commitcomment-2403679
[promo] Fix 1.8 syntax error in promotions/actions/create_adjustment_spec