fix: prevent unnecessary flattening of headers during convertRes

[cboar]

I ran into an issue where multiple Set-Cookie headers were being joined into a single header, which is not valid. I found that the convertRes function flattens all headers right before constructing the InternalResult. Since InternalResult allows string[] headers, I rewrote the parseHeaders to keep these as is. Since the method is also used elsewhere, I renamed the original implementation to flattenHeaders to better reflect its behavior.

Partially fixes #962

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[cboar]

@sommeeeer I see #977 solves a similar issue exclusively for Location, but this also seems to affect Set-Cookie when multiple cookies are set.

For context, I am using a custom wrapper built for AWS ALB responses (not API Gateway), it has been helpful to not join the headers returned from InternalResult.

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commit: 2e3faf1