Ignore "order_completed" flash on rails 4.1

The code to ignore the "order_completed" flash in base_helper isn't working on rails 4.1. If you refer to the source you'll see that keys are being stringified on assignment and because of that we need to ignore flashes based on strings instead of symbols. 

Relavant link to see how keys are stringified in 4.1 when new flashes are assigned:
https://github.com/rails/rails/blob/4-1-stable/actionpack/lib/action_dispatch/middleware/flash.rb