Configatron::Store#fetch with a falsey default should work as expected

[mtjhax]

Configatron::Store#fetch with a falsey default should set and return that default. At the moment it sets and returns nil, for example:

configatron.configure_from_hash(test_config: {})
=> {:test_config=>{}}
configatron.test_config.fetch(:enabled, false)
=> nil

One would expect the result to be false. This could be a breaking change for someone, in theory.