Recurse when calculating whether an init param is required

Having even one optional header means we have to explicitly provide them when constructing a response - that's not great.

For example:

```swift
return .ok(.init(body: .json(.init(foo: "bar"))))
```

Now, even when we don't want to provide any headers, we'll have to provide the param.

This should only happen when there is at least one _required_ header.