[8.x] Encode objects when casting as JSON

[halloei]

Currently one's not able to use objects with castAsJson():

ErrorException: PDO::quote() expects parameter 1 to be string, object given

The following code can be used to reproduce the bug:

    $this->assertDatabaseHas('table', [
        'column' => $this->castAsJson((object)[
            'foo' => 'bar',
        ]),
    ]);