netlify dev: correctly mirror Lambda's behaviour when returning synchronously

[Skn0tt]

As captured in netlify/functions#308, there's currently a difference in behaviour between ntl dev and production behaviour for functions that respond synchronously:

export const handler = (event) => {
  return { statusCode: 200, body: "test" }
}

While ntl dev will takes this as a literal response, a deployed function will discard this return value. Since that behaviour is controlled by Lambda, we should move ntl dev's behaviour in-line.

[Skn0tt]

I opened an upstream PR to lambda-local to address this: ashiina/lambda-local#220