fix(fetch): remove content-length header on redirect (nodejs#2022)

Fixes nodejs#2021

lib/fetch/constants.js

@@ -48,11 +48,17 @@ const requestCache = [
   'only-if-cached'
 ]
 
+// https://fetch.spec.whatwg.org/#request-body-header-name
 const requestBodyHeader = [
   'content-encoding',
   'content-language',
   'content-location',
-  'content-type'
+  'content-type',
+  // See https://github.com/nodejs/undici/issues/2021
+  // 'Content-Length' is a forbidden header name, which is typically
+  // removed in the Headers implementation. However, undici doesn't
+  // filter out headers, so we add it here.
+  'content-length'
 ]
 
 // https://fetch.spec.whatwg.org/#enumdef-requestduplex

test/fetch/issue-2021.js

@@ -0,0 +1,32 @@
+'use strict'
+
+const { test } = require('tap')
+const { once } = require('events')
+const { createServer } = require('http')
+const { fetch } = require('../..')
+
+// https://github.com/nodejs/undici/issues/2021
+test('content-length header is removed on redirect', async (t) => {
+  const server = createServer((req, res) => {
+    if (req.url === '/redirect') {
+      res.writeHead(302, { Location: '/redirect2' })
+      res.end()
+      return
+    }
+
+    res.end()
+  }).listen(0).unref()
+
+  t.teardown(server.close.bind(server))
+  await once(server, 'listening')
+
+  const body = 'a+b+c'
+
+  await t.resolves(fetch(`http://localhost:${server.address().port}/redirect`, {
+    method: 'POST',
+    body,
+    headers: {
+      'content-length': Buffer.byteLength(body)
+    }
+  }))
+})