Delete errorneous empty content length header (#53843)

fixes #53822

PR #53368 introduced a regression causing fetch with custom headers to fail in safari.
There are known issues with uncidi when a content-length: 0 header exists.

When performing a custom fetch in safari, this header is present.
When performing a custom fetch in chrome, this header is not present.




Co-authored-by: Zack Tanner <1939140+ztanner@users.noreply.github.com>

packages/next/src/server/lib/server-ipc/utils.ts

@@ -17,6 +17,12 @@ export const filterReqHeaders = (
   headers: Record<string, undefined | string | number | string[]>,
   forbiddenHeaders: string[]
 ) => {
+  // Some browsers are not matching spec and sending Content-Length: 0. This causes issues in undici
+  // https://github.com/nodejs/undici/issues/2046
+  if (headers['content-length'] && headers['content-length'] === '0') {
+    delete headers['content-length']
+  }
+
   for (const [key, value] of Object.entries(headers)) {
     if (
       forbiddenHeaders.includes(key) ||