[Translation] added method to expose collected message

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #2435 
| License       | MIT
| Doc PR        | symfony/symfony-docs#3386
I see you point. And following this logic, `getMessages(array())` should return empty list :)
Think, it's unnecessary to cover this case. If you don't mind, I would prefer to leave how it is now. Thanks.