through the PUT /transactions/{transactionId} method.
Restrict the PUT to changing only the category. The other existing
attributes should remain immutable.
Remove the body of the PUT response, it isn't required, and it was
returning a 204, which shouldn't have it.
This patch also extracts the CategoryName as a separate component on the
OpenAPI spec, so that it can be reused on the Transaction.
Issues #26 and #23
