[Bug] API Gateway v2 cookies response should be array

According to API Gateway v2 payload 2.0 docs (https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-develop-integrations-lambda.html) the 'cookies' should be array.
And below code is current handling logic for it.

https://github.com/vendia/serverless-express/blob/671f914429e9708bfb1ee6ba76b5947a917592df/src/event-sources/aws/api-gateway-v2.js#L65-L67

However, with the latest express (v4.17.1).
When user set cookie using `res.cookie(name, value)`, it set 'set-cookie' header as string at first time.
and convert it as array when using `res.cookie(name, value)` more than once.

So, we should insert Array check logic, and convert it to array when it is not.

Plus,
If we didn't delete 'set-cookie' header in response, API gateway send 'set-cookie' header twice.
So, for gateway v2 and payload 2.0 we should delete 'set-cookie' header field.

src/event-sources/aws/api-gateway-v2.js のリンクされた処理ロジックから始め、Express が1つの cookie を設定する場合と複数の cookie を設定する場合のレスポンス動作を比較します。完了条件は、API Gateway v2 が cookies を配列として受け取り、元の set-cookie ヘッダーが削除されて二重に送信されないことです。

索引モデルが issue の本文から書いたものです。