Add missing fields for category_id and category_name in the mail_chimp API submission

[rterbush]

There may be cleaner ways to do this, but this does fix the issue of missing data in the API call to ecomm_order_add().

[joshnuss]

thanks @rterbush!

btw was category_id causing an error? my understanding is that it is an optional field

[rterbush]

@joshnuss Yes, this was the error that was causing the failure. According to the MC docs for APIVERSION 1.3, these fields are not called out as optional. http://apidocs.mailchimp.com/api/rtfm/ecommorderadd.func.php