API Gateway Proxy: headers are case sensitive

By standard headers is case insensitive, but `APIGatewayProxyRequestEvent` is instantiated using a regular Map (probably a Hashmap) which causes headers to be case sensitive. I couldn't find the code where it is instantiated, so Idk exactly what is going on there.

This is a similar problem, but for Go language: https://github.com/aws/aws-lambda-go/issues/117