[fix] adapter-netlify: support for multivalue headers

[JeanJPNM]

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Fixes #1741 by moving set-cookie to multiValueHeaders whenever it sets more than one cookie.

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🦋 Changeset detected

Latest commit: 29c9c3c

The changes in this PR will be included in the next version bump.

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Name	Type
@sveltejs/adapter-netlify	Patch

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[benmccann]

This looks pretty good to me. The one thing I wonder though is if it's the case that Set-Cookie is the only multi-value header that will ever exist. I might be safer to iterate though and check which headers have multiple values rather than specifying a list of headers to check

[benmccann]

thanks!!