`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.

Start by tracing how the API Gateway V1 response converts input headers into multiValueHeaders, then compare that behavior with aws-serverless-express. Verify the expected response shape for the Cloudflare worker proxy, including whether headers, multiValueHeaders, or both should be emitted.

Vom Indexierungsmodell aus dem Issue-Text verfasst.