Type inference for HeaderBag could support has()

Right now calling this triggers an error as the returned header is string|null, but really after `->has($key)` returns true, we can assert that `->get($sameKey)` will be `string`. At least as long as there is no other impure call made in between like `->remove()` or `->replace()`

```php
if ($request->headers->has('foo')) {
    $v = strtolower($request->headers->get('foo'));
}
```