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

[JiaLiPassion]

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

newBluebirdPromise((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');returnPromise.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.