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.
Okay for this pull request. I did not know there are some plugins calling `Config.bool_value` directly. 

Still, this test cases look same with just calling `Config.bool_value`, so tests above 62 lines look enough.