Fix res.json support for string / null (#36186)

Continuation of #33592 with updates tests / changes.

Co-Authored-By: Balázs Orbán <info@balazsorban.com>

Co-authored-by: Balázs Orbán <info@balazsorban.com>
Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

packages/next/server/api-utils/node.ts

@@ -442,7 +442,7 @@ function sendJson(res: NextApiResponse, jsonBody: any): void {
   res.setHeader('Content-Type', 'application/json; charset=utf-8')
 
   // Use send to handle request
-  res.send(jsonBody)
+  res.send(JSON.stringify(jsonBody))
 }
 
 function isNotValidData(str: string): boolean {

test/integration/api-support/pages/api/json-null.js

@@ -0,0 +1,3 @@
+export default (req, res) => {
+  res.json(null)
+}

test/integration/api-support/pages/api/json-string.js

@@ -0,0 +1,3 @@
+export default (req, res) => {
+  res.json('Hello world!')
+}

test/integration/api-support/test/index.test.js

@@ -163,11 +163,27 @@ function runTests(dev = false) {
   })
 
   it('should support undefined response body', async () => {
-    const res = await fetchViaHTTP(appPort, '/api/undefined', null, {})
+    const res = await fetchViaHTTP(appPort, '/api/json-undefined', null, {})
     const body = res.ok ? await res.text() : null
     expect(body).toBe('')
   })
 
+  it('should support string in JSON response body', async () => {
+    const res = await fetchViaHTTP(appPort, '/api/json-string', null, {})
+    const body = res.ok ? await res.text() : null
+    expect(body).toBe('"Hello world!"')
+  })
+
+  it('should support null in JSON response body', async () => {
+    const res = await fetchViaHTTP(appPort, '/api/json-null')
+    const body = res.ok ? await res.json() : 'Not null'
+    expect(res.status).toBe(200)
+    expect(res.headers.get('content-type')).toBe(
+      'application/json; charset=utf-8'
+    )
+    expect(body).toBe(null)
+  })
+
   it('should return error with invalid JSON', async () => {
     const data = await fetchViaHTTP(appPort, '/api/parse', null, {
       method: 'POST',