For #5, fix the tests

- custom filter will nil everything, truncating is not permitted
- register filters/filter words with logger instance

features/step_definitions/filter_steps.rb

@@ -32,20 +32,23 @@
 end
 
 Given(/^I set filter words to include "([^"]*)"$/) do |filter_word|
-  TeeLogger::Filter.filter_words = [filter_word]
+  logger.filter_words = [filter_word]
 end
 
 Given(/^I register a custom filter$/) do
   class MyFilter < TeeLogger::Filter::FilterBase
     FILTER_TYPES = [String]
     WINDOW_SIZE  = 1
     def process(*args)
-      # Eat all arguments
-      return []
+      # Nil everything
+      args.each_with_index do |dummy, idx|
+        args[idx] = nil
+      end
+      return args
     end
   end
 
-  TeeLogger::Filter.register_filter(MyFilter)
+  logger.register_filter(MyFilter)
 end
 
 Given(/^I write a log message containing the word sequence "([^"]*)", "([^"]*)"$/) do |word1, word2|