Fix constructor-based checks for fake Date no longer pass after installing (#512)

Author: alexanderweiss

src/fake-timers-src.js

@@ -450,7 +450,7 @@ function withGlobal(_global) {
 
                 // ensures identity checks using the constructor prop still works
                 // this should have no other functional effect
-                this.constructor = Date;
+                this.constructor = NativeDate;
             }
 
             static [Symbol.hasInstance](instance) {

test/fake-timers-test.js

@@ -3203,6 +3203,9 @@ describe("FakeTimers", function () {
         // issue #510
         it("creates Date objects where the constructor prop matches the original", function () {
             const realDate = new Date();
+            Date = NOOP; // eslint-disable-line no-global-assign
+            global.Date = NOOP;
+
             const date = new this.clock.Date();
 
             assert.equals(date.constructor.name, realDate.constructor.name);