Respond with 403 if Authorization header is not present

:koala: :bug: 

This PR fixes an issue where the API would return 200 with an empty response body if there was no Authorization header in the request. This is causing the Setup guide to fail silently for some users.

/cc @jwswj @maximeprades @ebizmarts
@danielbreves I've created a follow up for that customer yesterday.
I'll wait to see if he gets back to me with data in regards why preg_match fails for him.