[promo] Fix 1.8 syntax error in promotions/actions/create_adjustment_spec
Introduced datepicker_field_value helper method for datepicker fields

Fixes #2405
Fix class name of promo :expires_at field

Fixes #2405
Freeze to rspec-rails 2.12.0 due to this commit:

https://github.com/rspec/rspec-rails/commit/09793f885637ee4d6f5cc2c10b4f60b078acb9b5#commitcomment-2403679