Binding promises breaks unhandled rejection reporting

``` javascript
require('bluebird')
  .reject(new Error('error'))
  .bind(null);
```

With bluebird 3.0.5, this prints nothing. Only if the `bind(null)` line is removed it will throw an unhandled rejection error.

With bluebird 2.10.2, it is working fine in both cases and the unhandled rejection is always reported.
