fixed bool_value

Fluent::Config.bool_value(nil) returned nil in the v0.10.46, but it has returned true since v0.10.47.
This patch fixs this problem.
```rb
      sub_test_case 'v1_config use' do
        test 'nil' do
```

to

```rb
      test 'with v1_config' do
```

is enough.