Response.headers must be immutable

WPT expects:
- ❌ Response.headers.append should throw TypeError (we don't throw anything)
- ✅ Response.headers.get should not return the added header (we do this correctly)

Something seems whacky to me about the way we implemented immutability. If I print out a `Headers` object from a Response, I see:

```
Headers(1) {
'content-type' => 'text/plain;charset=UTF-8',
[immutable]: false
}
```

Seems sus?