Missing multiValueHeaders in APIGatewayProxyResponseEvent #227

Hi,

I'm missing multiValueHeaders in com.amazonaws.services.lambda.runtime.events.APIGatewayProxyResponseEvent, It makes it somewhat cumbersome when one eg. wants to set multiple cookies with "non-compatible" properties that require separate header entries. To work around it, we currently have to return custom objects instead that where we set multiValueHeaders - would be great if we didn't have to!

com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent already supports it.

Thanks,
Daniel