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.
Using `data` method is better. See this test: https://github.com/fluent/fluentd/blob/61fbdb770c7c4b46e77c293d7dbe5fa568918e66/test/test_formatter.rb#L234