`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.