Add a null process to Preprocessor

Fixes Andrew's 'refactor'.

lib/bookbinder/preprocessing/preprocessor.rb

@@ -1,22 +1,26 @@
 module Bookbinder
   module Preprocessing
     class Preprocessor
-      def initialize(*processes, default: nil)
+      class NullProcess
+        def preprocess(*)
+        end
+      end
+
+      def initialize(*processes)
         @processes = processes
-        @default = default
       end
 
       def preprocess(objs, *args, &block)
         objs.group_by { |obj|
-          processes.detect ->{ default } { |process| process.applicable_to?(obj) }
+          processes.detect ->{ NullProcess.new } { |process| process.applicable_to?(obj) }
         }.each do |process, objs|
           process.preprocess(objs, *args)
         end
       end
 
       private
 
-      attr_reader :processes, :default
+      attr_reader :processes
     end
   end
 end

spec/lib/bookbinder/preprocessing/preprocessor_spec.rb

@@ -25,26 +25,23 @@ module Preprocessing
         preprocessor.preprocess(objs, 'extra', 'args')
       end
 
-      it "applies the default process to objects that aren't applicable to any process" do
+      it "does nothing to objects that aren't applicable to any process" do
         objs = [
           Object.new,
           Object.new,
           Object.new,
           Object.new,
         ]
 
-        process_1 = double('default process')
-        process_2 = double('some other process')
+        process_1 = double('some other process')
+        process_2 = double('second process')
 
-        preprocessor = Preprocessor.new(process_2, default: process_1)
-
-        block = ->(*){}
+        preprocessor = Preprocessor.new(process_1, process_2)
 
-        allow(process_1).to receive(:applicable_to?) { false }
-        allow(process_2).to receive(:applicable_to?) { |obj| objs[2..3].include?(obj) }
+        allow(process_1).to receive(:applicable_to?) { |obj| objs[2..3].include?(obj) }
+        allow(process_2).to receive(:applicable_to?) { false }
 
-        expect(process_1).to receive(:preprocess).with(objs[0..1], 'extra', 'args')
-        expect(process_2).to receive(:preprocess).with(objs[2..3], 'extra', 'args')
+        expect(process_1).to receive(:preprocess).with(objs[2..3], 'extra', 'args')
 
         preprocessor.preprocess(objs, 'extra', 'args')
       end