HasOne relations hydrated by id using ItemHydrator are not correctly serialized

[Jasper Zonneveld]

Detailed description

$itemHydrator = new ItemHydrator(new TypeMapper());

$item = $itemHydrator->hydrate(new ItemWithHasOneRelation(), ['hasone' => 1]);

$item->toJsonApiArray();

Result:

[
    'type' => 'foo-bar',
    'relationships' => [
        'hasone' => [
            'data' => null
        ]
    ]
]

Expected:

[
    'type' => 'foo-bar',
    'relationships' => [
        'hasone' => [
            'data' => [
                'type' => 'foo-baz',
                'id' => 1
            ]
        ]
    ]
]