server-adapter: prevent to replace correct swr header

[Tietew]

server-adapter replaces stale-while-revalidate in Cache-Control header.
ref. https://github.com/serverless-stack/open-next#workaround-nextserver-does-not-set-correct-swr-cache-headers

When getServerSideProps sets correct swr, for example:

export const getServerSideProps = async ({ res }) => {
  // fetch my data
  res.setHeader('Cache-Control', 'max-age=60, stale-while-revalidate=86400');
  return { props: /* my data */ }
};

server-adapter generates incorrect Cache-Control:

Cache-Control: max-age=60, stale-while-revalidate=2592000=86400

This PR fixes to prevent replacing stale-while-revalidate=<duration>.

Test:

> 'max-age=60, stale-while-revalidate'.replace(/\bstale-while-revalidate(?!=)/, "stale-while-revalidate=2592000")
'max-age=60, stale-while-revalidate=2592000'
> 'max-age=60, stale-while-revalidate=86400'.replace(/\bstale-while-revalidate(?!=)/, "stale-while-revalidate=2592000")
'max-age=60, stale-while-revalidate=86400'
> 'max-age=60, stale-while-revalidate, must-revalidate'.replace(/\bstale-while-revalidate(?!=)/, "stale-while-revalidate=2592000")
'max-age=60, stale-while-revalidate=2592000, must-revalidate'
> 'max-age=60, stale-while-revalidate=86400, must-revalidate'.replace(/\bstale-while-revalidate(?!=)/, "stale-while-revalidate=2592000")
'max-age=60, stale-while-revalidate=86400, must-revalidate'

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Name	Status	Preview	Comments	Updated (UTC)
open-next	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 5, 2023 4:47pm

[jayair]

Just checking if you posted about this on #open-next on Discord?

[khuezy]

LGTM, thanks!

[khuezy]

Sorry for the long delay. I'll try to merge this by the end of week, there's some critical infra PRs that need to go in ahead of this.
Don't worry about the conflict, I'll address it along w/ some unit tests.

[khuezy]

Added some unit tests but haven't set up the GH actions yet, waiting for #208 first

[khuezy]

Hmm, sorry for the random pushes, GH was glitching... it's behaving correctly now

[khuezy]

Hi @Tietew I merged this PR but had to revert b/c my commits on top of it was causing the app to add quotes in the URL... not sure what was causing that, it might have been from a strange rebase... anyways your commit is in the git history so thanks for your contribution. I'll cherrypick the code in this PR: #214

Edit: the code was fine, AWS deployment was buggy. I took down the app and redeploy and it's fine... very strange.

[Tietew]

@khuezy Thanks much for your work!