Expose multiple headers with the same name in responses

[nviliunov-evolution-throwaway]

PR Checklist (not all items are relevant to all PRs)

• [x] Added unit-tests (for runtime code)
• [ ] Added bootstrapped code + smoke tests (when the rendering logic is modified)
• [ ] Added build-plugins integration tests (when reflection loading is required at codegen-time)
• [ ] Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)
• [ ] Updated dynamic module to match generated-code behaviour
• [ ] Added documentation
• [ ] Updated changelog

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