content-type and other headers with number strings are being converted to numbers

`@vendia/serverless-express@4.5.2`

```typescript
app.post('/webhook', async (req, res) => {
const { event, context } = getCurrentInvoke()
console.log(util.inspect({ event, req }, false, null))
}

// event.headers['content-type] = "420"
// req.headers['content-type] = 420
```

This is causing TS validation errors.
Any idea if this is something I can disable in express?