Scalar Values Wrapped as Lists in get_message_body()

[ronlo04]

I am encountering an issue where scalar values in my request body are being wrapped in lists when using the get_message_body() function from the RAPIClient library. This behavior results in a 400 Bad Request error from the API endpoint, as it expects scalar values but receives lists instead.

The problem seems to stem from how get_message_body() handles the serialization of data. Even when scalar values are provided, they are being converted into lists, which causes issues with the API’s expected input format.

Steps to Reproduce:

Here's a simple demo to reproduce the issue:

# Sample data
data_batch <- list(
  list(
    id = 101,
    name = "Item A",
    quantity = 10,
    price = 5.99,
    description = "Description of Item A"
  ),
  list(
    id = 102,
    name = "Item B",
    quantity = 20,
    price = 3.49,
    description = "Description of Item B"
  )
)


#### Expected Behavior:
The values like `id`, `name`, `quantity`, and `price` should remain as scalar values in the request body. For example:

```json
[
  {
    "id": 101,
    "name": "Item A",
    "quantity": 10,
    "price": 5.99,
    "description": "Description of Item A"
  },
  {
    "id": 102,
    "name": "Item B",
    "quantity": 20,
    "price": 3.49,
    "description": "Description of Item B"
  }
]

Actual Behavior:

The scalar values are being wrapped in lists, resulting in the following incorrect request body:

[
  {
    "id": [101],
    "name": ["Item A"],
    "quantity": [10],
    "price": [5.99],
    "description": ["Description of Item A"]
  },
  {
    "id": [102],
    "name": ["Item B"],
    "quantity": [20],
    "price": [3.49],
    "description": ["Description of Item B"]
  }
]

It seems that the get_message_body() function may be handling scalar values as list. From what I understand, the auto_unbox option could be involved in this behavior, causing scalar values to be unnecessarily wrapped in lists.

Is there a way to ensure that scalar values are not wrapped in lists during the request body preparation? Cheers