Sometimes, parts of the application pass in the locale as a string, not a symbol. This was causing the translate_accelerator to cache two versions of the locale separately: one cache for the symbol version, and one cache for the string version. For example, in a running production process:
```
irb(main):001:0> I18n.instance_variable_get(:@loaded_locales)
=> [:en, "en"]
```
This commit ensures the `locale` key is always converted to a symbol, and adds a spec to ensure the same locale cannot appear twice in `@loaded_locales`
1bc1a439ee