Added a simple getter for the output in the response class

Inside a UnitTest it would be necessary to check contents of the output with assertions. By adding a simple getter to the output class, this would be possible. An example would be something like this:

```php
class ControllerAccountWishListTest extends OpenCartTest {	
	public function testAddingAProductToTheAccountWishList() {
		
		// load the the wishlist controller within accout folder
		$controller = $this->loadControllerByRoute("account/wishlist");
		
		// set some test params for your request
		$this->request->post['product_id'] = 123;
		
		$controller->add();
		
		// get the response
		$output = $this->response->getOutput();
				
		// write some custom assertions for the ouput to verify the test
                // ... 
	}	
}
```

More can be found [here](https://github.com/openmandragora/opencart-test-suite)...