get returns reference to inner array

This is perhaps expected, but is it correct?

```javascript
	it( 'results of prevous get should not be modified by future set', () => {
		let map = new Multimap( [['a',1]]);
		
		let aList = map.get('a');
		assert.strictEqual(aList.length, 1);
		
		map.set('a', 2);
		
		// aList.length is now 2
		assert.strictEqual(aList.length, 1);
	});
```

