Adjust POST /requests spec

[gr2m]

current state is at
http://docs.accountjsonapi.apiary.io/#reference/requests/request-collection/create

source is here:
https://github.com/hoodiehq/account-json-api/blob/master/apiary.apib#L725

---

• start a PR
• Add 201 response: links.self is optional. If no resource is persisted in the database, it won't be set (not relevant anymore, see comment below)
• specify a response if request handler is not implemented (403 maybe)?
• review & merge

[patriciagarcia]

As discussed with @gr2m, we'll always return links.self but until the persistence layer for requests is implemented, GET /requests/<request_id> will return 404.

• Add 201 response: links.self is optional. If no resource is persisted in the database, it will not be set