Categorize transaction

Updates the category associated with a transaction. Only transactions for which isCategorizable is set to true support this operation. The id is taken from the list exposed on /categories and cannot be one of the top-level (parent) categories. To de-categorize a transaction, set the entire data key to null. An HTTP 204 is returned on success. The associated category, along with its request URL is also exposed via the category relationship on the transaction resource returned from /transactions/{id}.