[fix] Use headers.raw() on split_headers (#4096)

* use headers.raw() on `split_headers`

* add changeset

Author: JeanJPNM

Diff for: .changeset/three-elephants-breathe.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-netlify': patch
+---
+
+Fix incorrect set-cookie header handling on adapter-netlify

Diff for: packages/adapter-netlify/src/handler.js

@@ -73,7 +73,8 @@ function split_headers(headers) {
 
 	headers.forEach((value, key) => {
 		if (key === 'set-cookie') {
-			m[key] = value.split(', ');
+			// @ts-expect-error (headers.raw() is non-standard)
+			m[key] = headers.raw()[key];
 		} else {
 			h[key] = value;
 		}