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.
In past, there are no way except for using `Config.bool_value` directly (or others) to parse parameter values in subsections.
(Now, we have `config_section`, but it is relatively new API, so we cannot expect that all plugin developers use it.)

So, compatibilities of these methods should be tested.