Spy passes through calling with new (#1626)

This changes the spy behavior so that if the spy is `calledWithNew()` then
the original function will also get called with `new`.

This was throwing an error if you tried spying on an ES6 class object.

fixes #1265

lib/sinon/spy.js

@@ -17,6 +17,7 @@ var push = Array.prototype.push;
 var slice = Array.prototype.slice;
 var filter = Array.prototype.filter;
 var ErrorConstructor = Error.prototype.constructor;
+var bind = Function.prototype.bind;
 
 var callId = 0;
 
@@ -190,11 +191,18 @@ var spyApi = {
         try {
             this.invoking = true;
 
-            returnValue = (this.func || func).apply(thisValue, args);
-
             var thisCall = this.getCall(this.callCount - 1);
-            if (thisCall.calledWithNew() && typeof returnValue !== "object") {
-                returnValue = thisValue;
+
+            if (thisCall.calledWithNew()) {
+                // Call through with `new`
+                // eslint-disable-next-line new-parens
+                returnValue = new (bind.apply(this.func || func, [thisValue].concat(args)))();
+
+                if (typeof returnValue !== "object") {
+                    returnValue = thisValue;
+                }
+            } else {
+                returnValue = (this.func || func).apply(thisValue, args);
             }
         } catch (e) {
             exception = e;

test/spy-test.js

@@ -439,6 +439,16 @@ describe("spy", function () {
             assert(called);
         });
 
+        it("passes 'new' to underlying function", function () {
+            function TestClass() {}
+
+            var SpyClass = createSpy.create(TestClass);
+
+            var instance = new SpyClass();
+
+            assert(instance instanceof TestClass);
+        });
+
         it("passs arguments to function", function () {
             var actualArgs;