Fix toString() output

The output when just inheriting was
function() { [native code] }
vs
function Date() { [native code] }
in the real Date object.

I had to make a choice between having the same function identity and function output and opted for the latter,
since that is what the mother sinon project targets.

src/fake-timers-src.js

@@ -456,6 +456,10 @@ function withGlobal(_global) {
             };
         }
 
+        ClockDate.toString = function toString() {
+            return NativeDate.toString();
+        };
+
         // noinspection UnnecessaryLocalVariableJS
         /**
          * A normal Class constructor cannot be called without `new`, but Date can, so we need

test/fake-timers-test.js

@@ -3358,8 +3358,8 @@ describe("FakeTimers", function () {
             });
         });
 
-        it("mirrors toString", function () {
-            assert.same(this.clock.Date.toString, Date.toString);
+        it("mirrors toString output", function () {
+            assert.same(this.clock.Date.toString(), Date.toString());
         });
     });