fetch option headers difference - array of pairs

### firefox (133) fetch

```typescript
fetch("/path", { headers: [["A", "B"], ["C", "D"] })
```
results in the following headers on the request:
```
A: B
C: D
```

### whatwg-fetch polyfill (3.6.20)

```typescript
fetchPolyfill("/path", { headers: [["A", "B"], ["C", "D"]] })
```
results in the following headers on the request:
```
0: A,B
1: C,D
a: B
c: D
```
