Use i18n for submit button for issue comments

Normally when an en-GB.yml translation is missing, rails falls back to en.yml. But when
using the submit helpers, if the en-GB translation is missing, rails knows how to create
a fallback like 'Create {model_name}' without touching the en.yml file. This string might
then be different from what the test expects, e.g. 'Add Comment'.

So it's important to set the language headers, to avoid phantomjs from picking up your desktop
preferences in this specific case.