public function testKeyIsEmpty()
{
$collection = new Collection([1 => 'test2', 0 => 'test']);
$this->assertEquals(new Collection([0 => 1, 1 => 0]), $collection->keys());
$collection = new Collection(['' => 'test']);
$value = $collection->keys()[0];
$this->assertTrue('' === $value);
$collection = new Collection(['test']);
$value = $collection->keys()->toArray()[0];
$this->assertTrue(0 === $value);
}
