Initialize Logger instanance variables, if not defined.

Fixes #428.

Author: gkellogg

lib/rdf/util/logger.rb

@@ -18,6 +18,9 @@ module Logger
     # @option options [Logger, #<<] :logger
     # @return [Logger, #write, #<<]
     def logger(**options)
+      # Guard against undefined instance variables, which may be a warning if used.
+      @logger = nil unless instance_variable_defined?(:@logger)
+      @options = nil unless instance_variable_defined?(:@options)
       logger = options.fetch(:logger, @logger)
       logger = @options[:logger] if logger.nil? && @options
       if logger.nil?