Blacklist fetch response headers when serialised for initial fetch

[tanhauhau]

Describe the problem

when serialise the fetch response into <script type="application/json" data-type="svelte-data">, it includes the fetch response header, which includes headers that is always changed:

• Date

which makes the html content hash different, even though the api response body and svelte rendered content is the same

Describe the proposed solution

blacklist some of the response header when serialised

Alternatives considered

use serverFetch hook to overwrite fetch behavior on server, and remove the Date response header

Importance

would make my life easier

Additional Information

No response

[benmccann]

Blacklisting might be a bit brittle. E.g. Google adds X-Cloud-Trace-Context which is also a unique header

Unfortunately, the alternative of using serverFetch doesn't work because it only is for external fetches (there's a PR out to rename it to externalFetch to make that clearer #2110)

[benmccann]

I found an old message in Discord where you mentioned this with more detail:

i noticed for sveltekit, that the calculation of etag is based on the response body,
which includes the serialised response from fetch, which one of the headers is Date, which will never be the same.
so i never got the same etag if i have fetch, tho the response body and rendered html is exactly the same