[WIP] enhance shouldRejected reason

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