Upstream added a capital 'T' to the 'Translation missing' message in https://github.com/ruby-i18n/i18n/commit/c5c6e753f3. This caused our translate accelerator patch to diverge, and the change in case affected a number of our specs. This commit updates the translate accelerator to match the upstream casing, and introduces a spec to detect future divergence.
This happened only for languages other than "en" and when `I18n.t` was called without any interpolation keys. The lib still tried to interpolate keys because it interpreted the `overrides` option as interpolation key.
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.