[5.5] Handle a case in Collection::where() for objects that can be casted to string

[themsaid]

$c = new Collection([['v' => 1], ['v' => new \Illuminate\Support\HtmlString('hello')]]);

$c->where('v', 'hello')->values()->all();

The changes in this PR will ensure the comparison occurs when the values are both strings or castable to strings.