Simple formatter which only displays the message.

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

This method is invoked when a log event occurs

    # File activesupport/lib/active_support/clean_logger.rb, line 97
97:     def call(severity, timestamp, progname, msg)
98:       "#{String === msg ? msg : msg.inspect}\n"
99:     end