fix(ssr): duplicate custom headers

[ematipico]

Changes

Closes #12419
Closes PLT-2669

This PR changes the merging strategy of the headers in SSR when we render an error page. It relies on a Map to remove the duplicates.

Testing

Added a new test

Docs

N/A

[changeset-bot]

🦋 Changeset detected

Latest commit: 12ed91c

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[codspeed-hq]

CodSpeed Performance Report

Merging #12518 will not alter performance

_{Comparing fix/duplicate-headers (12ed91c) with main (b140a3f)}

Summary

✅ 6 untouched benchmarks

[ascorbic]

I'm not sure if it's likely to come up here, but duplicate headers are legal, so the original headers may already have duplicates that would be stripped here

[ematipico]

I'm not sure if it's likely to come up here, but duplicate headers are legal, so the original headers may already have duplicates that would be stripped here

They are, but once they are "compiled", the key is one their values are concatenated into a single string divided by comma, correct?

This means that the original response will already have the duplicate headers "compiled", correct?

[ascorbic]

I thought that just applied to get(), but I see it's for iterators too, so I guess not a problem