Rack::CommonLogger forwards every request to the given app, and logs a line in the Apache common log format to the logger.

If logger is nil, CommonLogger will fall back +rack.errors+, which is an instance of Rack::NullLogger.

logger can be any class, including the standard library Logger, and is expected to have either write or +<<+ method, which accepts the CommonLogger::FORMAT. According to the SPEC, the error stream must also respond to puts (which takes a single argument that responds to to_s), and flush (which is called without arguments in order to make the error appear for sure)