Update 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 error can be reproduced, for example, when you try to unsubscribe someone of a list where it is already not subscribed.
Merge pull request #1 from gbpenha/patch-1

Bug found and fixed in the file ActiveResource.php