fix: allow unions with several object classes

[nikophil]

before this PR, an error would have occurred when trying to map a property which is an union of several objects.

This code:

private Foo|Bar $prop

was generating this mapper:

if (\AutoMapper\MapperContext::isAllowedAttribute($context, 'prop', isset($value->prop))) {
    $value_1 = $value->prop;
    if (is_object($value->prop)) {
        // map form Foo
    }
    if (is_object($value->prop)) {
        // map form Bar
    }
    $result['prop'] = $value_1;
}

so it never tried to handle case when $prop is Bar

now, it adds an instanceof:

if (\AutoMapper\MapperContext::isAllowedAttribute($context, 'prop', isset($value->prop))) {
    $value_1 = $value->prop;
    if (is_object($value->prop) && $value->prop instanceof Foo) {
        // map form Foo
    }
    if (is_object($value->prop) && $value6->prop instanceof Bar) {
        // map form Bar
    }
    $result['prop'] = $value_1;
}