[CartBundle] Fixed event used in CartListener and added specs

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

The dispatched event in ```addItem``` and ```removeItem``` is CartItemEvent instead of CartEvent, it uses the getItem method only available in CartItemEvent