[BUGFIX beta] Remove false attribute from Ember.View

In IE8 (or older), `jQuery.fn.prop` always update DOM against our
expectation.(This is a jQuery bug but it couldn't fix)
* ref: http://bugs.jquery.com/ticket/13558

So the following test is failed due to `view.$().attr('foo')` returns `"false"`:
* https://github.com/emberjs/ember.js/blob/a449d445/packages/ember-views/tests/views/view/attribute_bindings_test.js#L126
![2014-01-04 23 24 13](https://f.cloud.github.com/assets/290782/1845350/21b379b0-7583-11e3-85d6-24ac03cf3889.png)

To avoid this issue, `removeAttr` should be called when `null`, `undefined` or `false` is given as value.

But this way contains incompatible changes:

Before:
``` javascript
view.set('checked', false);
view.$().prop('checked'); //=> false
```

After:
``` javascript
view.set('checked', false);
view.$().prop('checked'); //=> undefined
```

Would you mind this change?
Or, What do you know any cool idea...?