where 方法，可以像数据库查询方法where()一样通过给定键值对过滤集合
```
$collection = collect([
['product' => 'Desk', 'price' => 200],
['product' => 'Chair', 'price' => 100],
['product' => 'Bookcase', 'price' => 150],
['product' => 'Door', 'price' => 100],
]);
$result = $collection->where('price', 100)->all();
/*
[
1 =>['product' => 'Chair', 'price' => 100],
3 =>['product' => 'Door', 'price' => 100],
]
*/
```
