Difficult to use when multiple Set-Cookie received

When there are duplicate headers, http-parser concatenates them with ', '. This makes it difficult to parse the individual cookies, e.g.:

```
Key1=Value1; path=/; expires=Fri, 19-Jul-24 03:30:12 GMT, Key2=Value2; path=/
```

Splitting on ', ' won't work. Though it can be parsed, but for other headers (maybe non-standard ones) it'll become impossible. If it's too late to make the value a list or something, what about a user-provided header callback that will run on each complete header line?