Don't proxy middleware if host is the same (#31180)

This changes the check for whether a rewrite is internal or if it should be proxied. Currently it checks if `protocol` is unset, which is only the case for relative URLs or localhost. This means that requests where there is a hostname set, or if localhost is specified in another way such as `127.0.0.1`, then it will be proxied, which potentially causes a proxy loop or ssl error. This PR changes the test so that it also checks if the hosts match, and only proxies if they are different.

Fixes #31179 

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

packages/next/server/next-server.ts

@@ -1241,7 +1241,12 @@ export default class Server {
               query: parsedUrl.query,
             })
 
-            if (parsedDestination.protocol) {
+            if (
+              parsedDestination.protocol &&
+              (parsedDestination.port
+                ? `${parsedDestination.hostname}:${parsedDestination.port}`
+                : parsedDestination.hostname) !== req.headers.host
+            ) {
               return proxyRequest(req, res, parsedDestination)
             }
 

test/integration/middleware/core/test/index.test.js

@@ -156,6 +156,16 @@ function rewriteTests(locale = '') {
     expect($('.title').text()).toBe('About Page')
   })
 
+  it(`${locale} should rewrite when not using localhost`, async () => {
+    const res = await fetchViaHTTP(
+      `http://localtest.me:${context.appPort}`,
+      `${locale}/rewrites/rewrite-me-without-hard-navigation`
+    )
+    const html = await res.text()
+    const $ = cheerio.load(html)
+    expect($('.title').text()).toBe('About Page')
+  })
+
   it(`${locale} should rewrite to Vercel`, async () => {
     const res = await fetchViaHTTP(
       context.appPort,