Bug found and fixed in the file ActiveResource.php

When something was done with the API and the status code returned was 422 (Unprocessable Entity), there was an error when trying to explode the $res variable and access the index 1, at line 215.

So, I put a verification for checking for the 422 status and return an error.

This bug can be reproduced, for example, when you try to unsubscribe someone of a list where it is already not subscribed.