Handler events.APIGatewayProxyRequest Cognito fields empty

I've deployed an API Gateway that calls a lambda_proxy handler. My handler function receives in a context and an `events.APIGatewayProxyRequest`. Inside the request, there are fields for the Cognito Identity ID and Pool ID, but both are empty. This may be related to Issue #106. While I saw a workaround to manually verify JWTs, that would seem to largely defeat the purpose of API Gateway Cognito authentication and certainly the request's cognito fields.

Code to reproduce the error:
```go
package main

func Handler(ctx context.Context, request events.APIGatewayProxyRequest) (events.APIGatewayProxyResponse, error) {
lc, _ := lambdacontext.FromContext(ctx)

fmt.Println("Context cognito identity id:", lc.Identity.CognitoIdentityID)
fmt.Println("Context Cognito pool:", lc.Identity.CognitoIdentityPoolID)
fmt.Println("Request context cognito id:", request.RequestContext.Identity.CognitoIdentityID)

return events.APIGatewayProxyResponse{}, nil
}

func main() {
lambda.Start(Handler)
}
```

I also dug into the aws-lambda-go/lambda/function.go Invoke() function and verified that the req.CognitoIdentityID and req.CognitoIdentityPoolId are both empty strings. I verified this using a deployed Lambda function that I tested using API Gateway and Postman (with valid JWT Cognito tokens) using Printlns also in the function.go Invoke() function.