Support Multiple Set-Cookie Headers

[metaskills]

API Gateway has supported multi-value parameters (https://aws.amazon.com/about-aws/whats-new/2018/10/amazon-api-gateway-adds-support-for-multi-parameters/) for some time. This includes response headers such as Set-Cookie for both REST API and HTTP API. Even ALB integrations support this:

• https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-develop-integrations-lambda.html
• https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#api-gateway-simple-proxy-for-lambda-output-format

This article (https://medium.com/@lancers/amazon-api-gateway-explaining-lambda-payload-version-2-0-in-http-api-24b0b4db5d36) does a really good job of breaking down the two AWS references docs.

The problem? Rack's HTTP spec (https://github.com/rack/rack/blob/master/SPEC.rdoc#the-headers-) for multiple values in a single Set-Cookie header is to join them with a \n new line character. We can not pass this as is to API Gateway or an ALB. Instead we pull the Set-Cookie header from the standard headers response and instead pass it via the proper interface of cookies or multiValueHeaders as needed.