fix: handle immutable headers returned by native fetch in middleware

[Kakadus]

Related Issue or Design Document

Close #588

On nodejs, the headers returned by fetch are immutable, following the whatwg standard. In the nextjs middleware, this leads to an TypeError when proxying e.g. POST login requests to kratos.

This clones the headers object before modification, preventing the error.

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

[changeset-bot]

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Latest commit: 832a8a1

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.02%. Comparing base (f3fad4d) to head (832a8a1).
⚠️ Report is 281 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #589       +/-   ##
===========================================
+ Coverage   42.43%   56.02%   +13.59%     
===========================================
  Files         136      174       +38     
  Lines        2008     3259     +1251     
  Branches      288      481      +193     
===========================================
+ Hits          852     1826      +974     
- Misses       1149     1358      +209     
- Partials        7       75       +68     

Components	Coverage Δ
@ory/elements-react	54.92% <ø> (+18.13%)	⬆️
@ory/nextjs	62.60% <ø> (-3.38%)	⬇️

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[Kakadus]

The formatting CI error is only on unrelated files. The codecov report also looks sketchy to me, as all lines are clearly executed / are in tests.

[jonas-jonas]

Thank you! Looks reasonable to me.