Allows requests from API user objects

[aprice42]

Fix for #112

As noted in that issue as of the v3 release of this library non-supported API calls such as this would no longer be possible:

$mailchimp_api = mailchimp_get_api_object('MailchimpLists');
$mailchimp_api->request('DELETE', '/lists/{list_id}/interest-categories/{interest_category_id}/interests/{interest_id}', $tokens);

The change in this PR would fix the BC break introduced in the v3 release.

[Berdir]

Didn't test but that's what I expect will work, yes.

[aprice42]

Great! My tests all worked as expected, going to merge this in and get this into another release.