BUG: lambdaurl.Wrap still does not add Content-Length headers?

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is.

Following https://github.com/aws/aws-lambda-go/issues/508,

> The [`http.ResponseWriter`](https://pkg.go.dev/net/http#ResponseWriter) implementation says the following thing about Content-Type header (and it actually has a note about Content-Length too).
>
> > If WriteHeader has not yet been called, Write calls
> > WriteHeader(http.StatusOK) before writing the data. If the Header
> > does not contain a Content-Type line, Write adds a Content-Type set
> > to the result of passing the initial 512 bytes of written data to
> > DetectContentType. Additionally, if the total size of all written
> > data is under a few KB and there are no Flush calls, the
> > Content-Length header is added automatically.
>
> As of right now the `lambdaurl.Wrap` implementation completely misses this . . .

which get closed as a part of https://github.com/aws/aws-lambda-go/releases/tag/v1.42.0

However a quick search of `Content-Length` in the change log of
https://github.com/aws/aws-lambda-go/compare/v1.41.0...v1.42.0

did not get any hits:

> 

Are the Content-Length headers being added now?

**Describe the solution you'd like**

Add the Content-Length headers, _properly_.

**Additional context**

For a handler like:

```go
mux.HandleFunc("/hello", func(w http.ResponseWriter, r *http.Request) {
w.WriteHeader(200)
_, _ = w.Write([]byte("Hello World!"))
})
```

I was able to get return from it if I try it locally, However, if using lambdaurl, I got empty returns, i.e., **`Content-Length: 0`**.

Please double-check. Thanks

lambdaurl.Wrap の実装から始め、ResponseWriter の動作を、issue に記載されている net/http の動作と比較します。提供されている /hello handler を lambdaurl 経由でローカルに再現し、小さなレスポンスが Content-Length: 0 ではなく、期待される Content-Length と body を返すことを確認します。

索引モデルが issue の本文から書いたものです。