fix: Consecutive `Set-Cookie` values being lost (#20)

.changeset/real-squids-know.md

···
+---
+'fetch-nodeshim': patch
+---
+
+Fix `Set-Cookie` list handling by capturing them with `Headers#append`

src/fetch.ts

···
 const headersOfRawHeaders = (rawHeaders: readonly string[]): Headers => {
   const headers = new Headers();
   for (let i = 0; i < rawHeaders.length; i += 2)
-    headers.set(rawHeaders[i], rawHeaders[i + 1]);
+    headers.append(rawHeaders[i], rawHeaders[i + 1]);
   return headers;
 };