Well it's not at all possible for PHP to return that structure. 

```
[
  header name => header value,
  header name => [header value1, header value2],
]
```

Keys with matching names aren't possible.

It could maybe return:

```
[
  header name => [
    header value,
    [header value1, header value2],
  ]
]
```

That structure could maybe go into setHeaders, but it's getting a bit disgusting.