fix(bluebird): fix #1112, bluebird chained callback should return a Bluebird Promise

[JiaLiPassion]

fix #1112.
The issue can be described as the following case.

new BluebirdPromise((resolve: any, reject: any) => {
      expect(Zone.current.name).toEqual('zone_A');
      resolve(1);
}).then((r: any) => {
      expect(r).toBe(1);
      expect(Zone.current.name).toEqual('zone_A');
      return Promise.resolve(2);
}).then((r: any) => {
      expect(r).toBe(2);
      expect(Zone.current.name).toEqual('zone_A');
});

So we should return a new bluebird promise in then callback.

[jrabbe]

Changes LGTM, would be great to have this merged and released as soon as convenient, @mhevery. Thank you.