feat( promise-test-helper ): enhance `shouldRejected` error message.

 ```js
 shouldRejected(this.fulfilledPromise).cath(function(error){});
 // => "Expected promise to be rejected but it was fulfilled"
 ```

 We don't know what's `this.fulfilledPromise` fulfilled **value**.

This commit to be following:

```js
 shouldRejected(this.fulfilledPromise).cath(function(error){});
 // => "Expected promise to be rejected but it was fulfilled: VALUE"
```
Merge pull request #1 from azu/enhance-shouldRejected-reason

Improve shouldRejected reason