Fix just headers not being applied in dev mode

packages/next/server/next-dev-server.ts

@@ -237,9 +237,9 @@ export default class DevServer extends Server {
     await this.loadCustomRoutes()
 
     if (this.customRoutes) {
-      const { redirects, rewrites } = this.customRoutes
+      const { redirects, rewrites, headers } = this.customRoutes
 
-      if (redirects.length || rewrites.length) {
+      if (redirects.length || rewrites.length || headers.length) {
         this.router = new Router(this.generateRoutes())
       }
     }

test/integration/custom-routes/test/index.test.js

@@ -885,4 +885,108 @@ describe('Custom routes', () => {
       })
     })
   })
+
+  describe('should load custom routes when only one type is used', () => {
+    const runSoloTests = isDev => {
+      const buildAndStart = async () => {
+        if (isDev) {
+          appPort = await findPort()
+          app = await launchApp(appDir, appPort)
+        } else {
+          const { code } = await nextBuild(appDir)
+          if (code !== 0) throw new Error(`failed to build, got code ${code}`)
+          appPort = await findPort()
+          app = await nextStart(appDir, appPort)
+        }
+      }
+
+      it('should work with just headers', async () => {
+        nextConfigContent = await fs.readFile(nextConfigPath, 'utf8')
+        await fs.writeFile(
+          nextConfigPath,
+          nextConfigContent.replace(/(async (?:redirects|rewrites))/g, '$1s')
+        )
+        await buildAndStart()
+
+        const res = await fetchViaHTTP(appPort, '/add-header')
+
+        const res2 = await fetchViaHTTP(appPort, '/docs/github', undefined, {
+          redirect: 'manual',
+        })
+        const res3 = await fetchViaHTTP(appPort, '/hello-world')
+
+        await fs.writeFile(nextConfigPath, nextConfigContent)
+        await killApp(app)
+
+        expect(res.headers.get('x-custom-header')).toBe('hello world')
+        expect(res.headers.get('x-another-header')).toBe('hello again')
+
+        expect(res2.status).toBe(404)
+        expect(res3.status).toBe(404)
+      })
+
+      it('should work with just rewrites', async () => {
+        nextConfigContent = await fs.readFile(nextConfigPath, 'utf8')
+        await fs.writeFile(
+          nextConfigPath,
+          nextConfigContent.replace(/(async (?:redirects|headers))/g, '$1s')
+        )
+        await buildAndStart()
+
+        const res = await fetchViaHTTP(appPort, '/add-header')
+
+        const res2 = await fetchViaHTTP(appPort, '/docs/github', undefined, {
+          redirect: 'manual',
+        })
+        const res3 = await fetchViaHTTP(appPort, '/hello-world')
+
+        await fs.writeFile(nextConfigPath, nextConfigContent)
+        await killApp(app)
+
+        expect(res.headers.get('x-custom-header')).toBeFalsy()
+        expect(res.headers.get('x-another-header')).toBeFalsy()
+
+        expect(res2.status).toBe(404)
+
+        expect(res3.status).toBe(200)
+        expect(await res3.text()).toContain('hello world!')
+      })
+
+      it('should work with just redirects', async () => {
+        nextConfigContent = await fs.readFile(nextConfigPath, 'utf8')
+        await fs.writeFile(
+          nextConfigPath,
+          nextConfigContent.replace(/(async (?:rewrites|headers))/g, '$1s')
+        )
+        await buildAndStart()
+
+        const res = await fetchViaHTTP(appPort, '/add-header')
+
+        const res2 = await fetchViaHTTP(appPort, '/docs/github', undefined, {
+          redirect: 'manual',
+        })
+        const res3 = await fetchViaHTTP(appPort, '/hello world')
+
+        await fs.writeFile(nextConfigPath, nextConfigContent)
+        await killApp(app)
+
+        expect(res.headers.get('x-custom-header')).toBeFalsy()
+        expect(res.headers.get('x-another-header')).toBeFalsy()
+
+        const { pathname } = url.parse(res2.headers.get('location'))
+        expect(res2.status).toBe(301)
+        expect(pathname).toBe('/docs/v2/advanced/now-for-github')
+
+        expect(res3.status).toBe(404)
+      })
+    }
+
+    describe('dev mode', () => {
+      runSoloTests(true)
+    })
+
+    describe('production mode', () => {
+      runSoloTests()
+    })
+  })
 })