Simple formatter which only displays the message.

Methods
C
Instance Public methods
call(severity, timestamp, progname, msg)

This method is invoked when a log event occurs

Source: show | on GitHub 

# File activesupport/lib/active_support/logger.rb, line 66
def call(severity, timestamp, progname, msg)
  "#{String === msg ? msg : msg.inspect}\n"
end