add ETag and Last-Modified to the handler.ts

[dohomi]

In the handler.ts you are adding following headers:

headers: {
      'Content-Type': ContentType,
      'Content-Disposition': ContentDisposition,
      'Cache-Control': 'max-age=31536000',
    },

It would be great if you could add ETag and Last-Modified as well to improve the caching options and increase a better SEO rating. Thanks!

[schickling]

Would you mind creating a PR for this?

[dohomi]

@schickling sorry I haven't had much time lately but now this issue gets more important for me. Is there any good guidance how to start? I want to deploy a website based on graphcool and I get downgraded on Google Pagespeed and YSlow due to this issue