Do not overwrite headers of StructuredReturn (#288)

* Do not overwrite headers of StructuredReturn
* Improve check for user-provided headers

lib/invoker.js

@@ -94,10 +94,13 @@ module.exports = function invoker(func) {
 
     // Check for user supplied body
     if (fnReturn.body !== undefined) {
-      if (typeof fnReturn.body === 'string') {
-        payload.headers['content-type'] = 'text/plain; charset=utf8';
-      } else if (typeof fnReturn.body === 'object') {
-        payload.headers['content-type'] = 'application/json; charset=utf8';
+      // Provide default content-type unless supplied by user
+      if (!payload.headers || !payload.headers['content-type']) {
+        if (typeof fnReturn.body === 'string') {
+          payload.headers['content-type'] = 'text/plain; charset=utf8';
+        } else if (typeof fnReturn.body === 'object') {
+          payload.headers['content-type'] = 'application/json; charset=utf8';
+        }
       }
       payload.response = fnReturn.body;
     } else if (typeof fnReturn === 'object' && !fnReturn?.body) {

test/fixtures/response-structured/index.js

@@ -0,0 +1,9 @@
+module.exports = function() {
+  return {
+    headers: {
+      'Content-Type': 'image/jpeg'
+    },
+    body: Buffer.from([0xd8, 0xff, 0xe0, 0xff, 0x10, 0x00, 0x46]),
+    statusCode: 201,
+  };
+};

test/test.js

@@ -589,3 +589,22 @@ test('Reads a func.yaml file for logLevel', t => {
         });
       }, errHandler(t));
 });
+
+test('Handles HTTP response provided as StructuredReturn', t => {
+  const func = require(`${__dirname}/fixtures/response-structured/`);
+  start(func)
+    .then(server => {
+      t.plan(3);
+      request(server)
+        .get('/')
+        .expect(201)
+        .expect('Content-Type', /jpeg/)
+        .end((err, res) => {
+          t.error(err, 'No error');
+          t.equal(typeof res.body, 'object');
+          t.ok(Buffer.isBuffer(res.body), 'body is a buffer');
+          t.end();
+          server.close();
+        });
+      }, errHandler(t));
+});