Process code review for ControllerConfiguration#initialize

See https://github.com/ReneB/arrthorizer/commit/77e2c9cf5ef302a14b0450bdf0898414d29b5a44
Boyscout: put default for Hash#fetch in a block

Responding to a code review at https://github.com/ReneB/arrthorizer/commit/77e2c9cf5ef302a14b0450bdf0898414d29b5a44
Replace Context.from_hash with explicit conversion method