[BUG] POST request not working with `Fetch::Request`

### Is there an existing issue for this?

- [X] I have searched the existing issues

### What version of `workers-rs` are you using?

0.3.0

### What version of `wrangler` are you using?

3.63.1

### Describe the bug

POST requests sent using `Fetch::Request` are never sent, and no error is emitted

### Steps To Reproduce

1. Use this example code:
```rs
let request = Request::new_with_init(
&webhook_url,
RequestInit::new()
.with_body(Some(serde_wasm_bindgen::to_value(
&json!({ "text": "hi"}),
)?))
.with_method(Method::Post),
)?;
Fetch::Request(request).send().await?;
```
2. Observe how the request is never sent, and code execution continues after the request is "sent" and awaited