Content is returned as { type: 'Buffer', data: [...] } with $return binding

With
```js
import { readBuffer } from '@wrote/read' // returns a buffer

export default async () => {
const body = await readBuffer('style.css')
context.res = {
body,
headers: {
'content-type': 'text/css',
},
}
}
```
and bindings
```json
{
"disabled": false,
"bindings": [
{
"authLevel": "anonymous",
"type": "httpTrigger",
"route": "style.css",
"direction": "in",
"name": "req",
"methods": [
"get"
]
},
{
"type": "http",
"direction": "out",
"name": "res"
}
]
}

```
my CSS file is served properly. When I change the bindings to use `$return`, I get served a buffer in serialised format:
```json
{
"type": "http",
"direction": "out",
"name": "$return"
}
```

```js
import { readBuffer } from '@wrote/read' // returns a buffer

export default async () => {
const body = await readBuffer('style.css')
return {
body,
headers: {
'content-type': 'text/css',
},
}
}
```
```
{"type":"Buffer","data":[46,103,111,111]}
```

---
Azure Functions Core Tools (2.1.748 Commit hash: 5db20665cf0c11bedaffc96d81c9baef7456acb3)
Function Runtime Version: 2.0.12134.0

Start by reproducing the shown Azure Functions HTTP response with the direct res binding and then with the $return binding, using the same Buffer content and CSS content type. Done means the $return response serves the Buffer as CSS content instead of exposing a serialized {"type":"Buffer","data":[...]} value.

Written by the indexing model from the issue text.