Next JS Api Routes - POST - Invalid Body response with request body payload

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the bug has not already been reported

### Fastify version

4.15.0

### Plugin version

9.2.0

### Node.js version

18.14.2

### Operating system

macOS

### Operating system version (i.e. 20.04, 11.3, 10)

13.2.1

### Description

I'm trying to create a NextJS API Route for a POST method and send a payload. I can get regular page routes to work fine, even an API route with a GET method, works fine. I can also get a regular POST API Route to work, without a payload being sent

I created a API route located here: /src/pages/api/user.js ... This is based on the example located here: https://github.com/fastify/fastify-nextjs/blob/master/pages/api/user.js

I then updated my start file with the following:

```
app.register(require("@fastify/nextjs")).after(() => {
app.next("/hello");
app.next("/api/user", { method: "POST" });
});
```

When I open Post Man and try to send a POST request with the payload:
```
{
"hello":"World"
}
```
the response I get is a 400 with "Invalid body" message.

I put together a SandBox located here:
https://codesandbox.io/s/lucid-rumple-ceetg2?file=/src/start.js:128-261

### Steps to Reproduce

1. Create API route located here: /src/pages/api/user.js
2. Update start.js for API route
3. Restart server
4. Make a POST request to your API route with a JSON body payload: http://localhost/api/user
5. See 400 Invalid Body response

### Expected Behavior

Expected behavior is a NextJS API route should handle a POST just like it does in NextJS. Am I doing something wrong?