POST requests that work with Axios throw CORS error with Redaxios

First, thank you for this awesome project! What a huge transfer size savings Redaxios is over Axios.

We have some code like below that works fine with Axios but fails with Redaxios:

```js
const response = await axios({
method: 'POST',
url: getApiUrl(),
data: {
...formData,
},
headers: {
'Content-Type': 'application/x-www-form-urlencoded',
},
});
```

We get:

> `Access to fetch at 'https://example.com/' from origin 'http://localhost:1212' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: It does not have HTTP ok status.`

[It seems possibly related to this question/answer](https://stackoverflow.com/a/37562814)

Riproduci l’esempio POST in un browser e analizza come Redaxios costruisce le richieste fetch e gestisce le opzioni relative al preflight. Confronta la richiesta con Axios e verifica che lo stesso POST codificato come form venga completato senza l’errore CORS segnalato.

Scritto dal modello di indicizzazione a partire dal testo della issue.