We intend to switch to the `:json` serializer, which will stringify all keys. However, we need a clean revert path. This commit ensures that our `_t` cookie handling works with both marshal (the current default) and json (the new default) serialization.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
