v2 HTTP API response header fix.

[aryounce]

This fix is related to #60 and possibly #75.

Description of changes:

The value for events.APIGatewayV2HTTPResponse.MultiValueHeaders is not being used by API Gateway v2 HTTP API instances and thus the returned headers are not passed through to the client. This change adds the value of each header in ProxyResponseWriterV2 to the events.APIGatewayV2HTTPResponse.Headers struct field.

• Extract v2 response headers from ProxyResponseWriterV2.headers.
• Include extracted headers in the returned events.APIGatewayV2HTTPResponse.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[aryounce]

I should also note that although MultiValueHeaders is a field in APIGatewayV2HTTPResponse the API Gateway documentation does not mention it as a part of v2 responses from Lambda integrations. See: Working with AWS Lambda proxy integrations for HTTP APIs, specifically: Lambda function response for format 2.0

To customize the response, your Lambda function should return a response with the following format.

{
    "cookies" : ["cookie1", "cookie2"],
    "isBase64Encoded": true|false,
    "statusCode": httpStatusCode,
    "headers": { "headername": "headervalue", ... },
    "body": "Hello from Lambda!"
}

[aryounce]

Closing as this is addressed by #99.