use_kwargs with fields.DateTime is parsed as string

When using `fields.DateTime` in a `use_kwargs` it is rendered as string in swagger/openapi.

```
@use_kwargs(
{
'from': fields.DateTime(
format="rfc",
default=datetime.datetime.now(),
description="The timestamp a vehicle should be available from"
)
}, location='query')
```

When using it inside the swagger editor, it is rendered as `string($date-time)`

**This is more conclusive for developers since the string has a specific format.**

Start by reproducing the issue with the shown use_kwargs and fields.DateTime example, then trace how that parameter is converted into Swagger/OpenAPI output. Confirm that the generated schema represents the timestamp with the specific date-time format rather than only a generic string.

Written by the indexing model from the issue text.