Send `fields.transactions` inner object in SEP-31 `POST /transactions` request

[JakeUrban]

When a SEP-31 receiving anchor doesn't require any per-transaction fields, the sending anchor should send a request like so:

{
   ...,
   "fields": {
        "transaction": {}
    }
}

However, the demo wallet sends:

{
    ...,
    "fields": {}
}

This is a minor issue, but strict servers will reject the first request because it technically doesn't comply with the schema. Our anchor platform currently rejects this request.