Use strict equality check within `hasProperty`

I wanted to check for the existence and equality of an object within the data attributes of an element and realized that the simple existence of the string representation of an object would make the test pass as well.

As an example this would pass with the current implementation:

```javascript
setFixtures('<div id="foo" div data-bar="[object Object]"></div>')
expect($('#foo')).toHaveData('bar', { 'answer': 42 });
```

The problem is caused by a simple equality check using `==` instead of a strict check using `===` in the value comparison of `hasProperty`.