`headers` missing on API Gateway V1 response

For some reason, the API Gateway V1 response does not include the `headers` field. It converts the input headers to `multiValueHeaders`.

This is a breaking change from `aws-serverless-express` and something that I am now patching manually with `patch-package` to get back the previous response object.

We have our own API Gateway-like proxy via a Cloudflare worker and CF (or browser) workers don't support multi value headers in the response, so at the very least it would be nice to have an option to just use `headers` or pass both of them as is to the proxy.

まず、API Gateway V1 response が input headers を multiValueHeaders に変換する仕組みを追跡し、その挙動を aws-serverless-express と比較します。Cloudflare worker proxy に期待される response shape を確認し、headers、multiValueHeaders、またはその両方を出力すべきかどうかも確認します。

索引モデルが issue の本文から書いたものです。