fix: ensure alb response header values are strings

__tests__/integration-alb.js

@@ -0,0 +1,37 @@
+const express = require('express')
+const serverlessExpress = require('../src/index')
+const {
+  makeEvent,
+  makeResponse
+} = require('../jest-helpers')
+
+describe('alb:express integration tests', () => {
+  test('reasponse headers are of type string', async () => {
+    const app = express()
+    const router = express.Router()
+    app.use('/', router)
+    const serverlessExpressInstance = serverlessExpress({ app })
+    router.get('/foo', (req, res) => {
+      res.send('123')
+    })
+    const event = makeEvent({
+      eventSourceName: 'alb',
+      path: '/foo',
+      httpMethod: 'GET',
+      headers: {}
+    })
+    const response = await serverlessExpressInstance(event)
+    const expectedResponse = makeResponse({
+      eventSourceName: 'alb',
+      body: '123',
+      headers: {
+        'content-length': '3',
+        'content-type': 'text/html; charset=utf-8',
+        'x-powered-by': 'Express',
+        etag: 'W/"3-QL0AFWMIX8NRZTKeof9cXsvbvu8"'
+      },
+      multiValueHeaders: undefined
+    })
+    expect(response).toMatchObject(expectedResponse)
+  })
+})

src/event-sources/aws/alb.js

@@ -52,7 +52,7 @@ const getResponseToAlb = ({
   const multiValueHeaders = !event.headers ? getMultiValueHeaders({ headers: responseHeaders }) : undefined
   const headers = event.headers
     ? Object.entries(responseHeaders).reduce((acc, [k, v]) => {
-      acc[k] = Array.isArray(v) ? v[0] : v
+      acc[k] = Array.isArray(v) ? String(v[0]) : String(v)
       return acc
     }, {})
     : undefined