Fix Airbrake::Sender#log to log messages when logger is not passed to options.

`Airbrake::Sender#log` is broken since https://github.com/airbrake/airbrake/commit/8e55c1f8ac81982b5bd34027ecb3c44a9382f82a. which required `options[:logger]`  to be passed - otherwise messages are not being logged. E.g. `Airbrake::Sender#send_to_airbrake` calls `log` few time with _no_ `:logger` passed and none of these messages/errors are being logged.