fix alias conflict when model is loading

variable $alias in model has a bug when loading, It is hard for me to  explain,the following example is the best explanation of this bug :


```
<?php
class TestsController extends AppController {
	public $uses = array('Blog.Post');

	public function lastPosts() {
		$this->loadModel('Blog.post');
		echo $this->Post->name // Post
		echo $this->Post->alias // post
		echo $this->post->name // Post
		echo $this->post->alias // post
		
		$this->Post->isBug = 'no';
		$this->post->isBug = 'yes';

		echo $this->post->isBug // yes
		echo $this->Post->isBug // yes

		echo $this->Post->find('all'); // array( 'post' => array( 'id' ....))
		
		pr($this->uses)  //  array('Blog.Post','Blog.post')
	}
?>
```