Handle the case when :restricted_characters is nil

Fixes #762

lib/paperclip/attachment.rb

@@ -441,6 +441,8 @@ module Paperclip
     def cleanup_filename(filename)
       if @options[:restricted_characters]
         filename.gsub(@options[:restricted_characters], '_')
+      else
+        filename
       end
     end
   end

test/attachment_test.rb

@@ -768,6 +768,14 @@ class AttachmentTest < Test::Unit::TestCase
     context "with specified regexp replacement" do
       setup do
         @old_defaults = Paperclip::Attachment.default_options.dup
+      end
+
+      teardown do
+        Paperclip::Attachment.default_options.merge! @old_defaults
+      end
+
+      context 'as another regexp' do
+        setup do
           Paperclip::Attachment.default_options.merge! :restricted_characters => /o/
 
           @file.stubs(:original_filename).returns("goood.png")
@@ -775,14 +783,25 @@ class AttachmentTest < Test::Unit::TestCase
           @dummy.avatar = @file
         end
 
-      teardown do
-        Paperclip::Attachment.default_options.merge! @old_defaults
-      end
-
         should "match and convert that character" do
           assert_equal "g___d.png", @dummy.avatar.original_filename
         end
       end
+
+      context 'as nil' do
+        setup do
+          Paperclip::Attachment.default_options.merge! :restricted_characters => nil
+
+          @file.stubs(:original_filename).returns("goood.png")
+          @dummy = Dummy.new
+          @dummy.avatar = @file
+        end
+
+        should "ignore and return the original file name" do
+          assert_equal "goood.png", @dummy.avatar.original_filename
+        end
+      end
+    end
   end
 
   context "Attachment with uppercase extension and a default style" do