fix(to-response): preserve multiple Set-Cookie when merging headers

[bytaesu]

This PR is based on @rastislavcore's work

• Replaces Fix #118 - toResponse overwrites multiple Set-Cookie headers #119
• Closes toResponse overwrites multiple Set-Cookie headers #118

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/better-call@137

commit: 58e13a9

[Copilot]

Pull request overview

Fixes toResponse() header merging so that multiple Set-Cookie headers are preserved (instead of being overwritten) when building/merging Response objects, addressing issue #118.

Changes:

• Add a copyHeaders() helper that append()s Set-Cookie while set()ing all other headers.
• Update toResponse() header merge points (existing Response branch and _flag: "json" branch) to use copyHeaders().
• Add tests covering multi-Set-Cookie preservation across the relevant merge scenarios.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/better-call/src/to-response.ts	Introduces copyHeaders() and updates merge logic to preserve multiple Set-Cookie values.
packages/better-call/src/to-response.test.ts	Adds a dedicated test block validating multi-Set-Cookie preservation and router header propagation.

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[better-release]

Successfully created backport PR for v1.3.x:

• [backport v1.3.x] fix(to-response): preserve multiple Set-Cookie when merging headers #142

[rastislavcore]

@bytaesu Can we produce release?