`HTTPFields` raw subscript behavior question

In current implementation (0.2.1), after you set a combined field value via the plain subscript method, you will get the combined value from raw subscript method too:

```swift
var fields = HTTPFields()
fields[.acceptLanguage] = "en-US, zh-Hans-CN"
print(fields[.acceptLanguage]!) // en-US, zh-Hans-CN
print(fields[raw: .acceptLanguage]) // ["en-US, zh-Hans-CN"]
```

But I expect the raw subscript method will return `["en-US", "zh-Hans-CN"]` in the above example. Is current behavior expected? Or it is a bug?

Start by reading the HTTPFields plain and raw subscript behavior described in the issue, then inspect the surrounding implementation and existing tests. Compare the observed combined-value result with the expected split-value result; done when the intended behavior is decided and covered by an appropriate test.

Written by the indexing model from the issue text.