Correctly mirror lambda's null-return behaviour (#220)

* add acceptance test

* fix: replicate lambda behaviour where returning synchronously is ignored

src/lambdalocal.ts

@@ -252,7 +252,7 @@ function _executeSync(opts) {
             if (result.then) {
                 result.then(ctx.succeed, ctx.fail);
             } else {
-                ctx.succeed(result);
+                ctx.succeed(null);
             }
         }
     } catch(err){

test/functs/test-func-synchronous.js

@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2018. Taimos GmbH http://www.taimos.de
+ */
+
+/*
+ * Lambda function used for basic test.
+ */
+exports.handler = function (event, context) {
+  return {
+    statusCode: 200,
+    body: "this response won't go anywhere!",
+  };
+};

test/test.js

@@ -324,6 +324,22 @@ describe("- Testing lambdalocal.js", function () {
             });
         });
     });
+    describe('* Synchronous return', function () {
+        it('should return null', function () {
+            var lambdalocal = require(lambdalocal_path);
+            lambdalocal.setLogger(winston);
+            return lambdalocal.execute({
+                event: require(path.join(__dirname, "./events/test-event.js")),
+                lambdaPath: path.join(__dirname, "./functs/test-func-synchronous.js"),
+                lambdaHandler: functionName,
+                callbackWaitsForEmptyEventLoop: false,
+                timeoutMs: timeoutMs,
+                verboseLevel: 1
+            }).then(data => {
+                assert.isNull(data)
+            })
+        });
+    });
     if (get_node_major_version() >= 2) {
         describe('* Promised Run', function () {
             var opts = {