Concatenating fields inconsistent with docs

According the [docs](https://github.com/apple/swift-http-types/blob/main/Sources/HTTPTypes/HTTPFields.swift#L161) setting multiple fields for the same header should append each field in a comma-separated list (unless it's a cookie). However

```swift
var headers = HTTPFields()
headers[.date] = "\(Date(timeIntervalSinceReferenceDate: 100.0))"
headers[.date] = "\(Date(timeIntervalSinceReferenceDate: -100.0))"
print(headers)
```

Only shows the last set date:

```
[(field: Date: 2000-12-31 23:58:20 +0000, next: 65535)]
```

Should it not show both dates? If you use `headers[values: .date] = [...]` it works as expected:

```
[(field: Date: 2001-01-01 00:01:40 +0000, next: 1), (field: Date: 2000-12-31 23:58:20 +0000, next: 65535)]
```

Start in Sources/HTTPTypes/HTTPFields.swift at the documentation linked in the issue, then reproduce the two assignments to the .date field and compare them with the values: subscript example. Done means the single-value assignment behavior matches the documented comma-separated handling for non-cookie fields, with the issue's date example demonstrating both values.

Written by the indexing model from the issue text.