fix exception when field is nil

lib/acts_as_stripped.rb

@@ -22,7 +22,7 @@ module InstanceMethods
     private
     def strip_fields
       strippable_attributes.each do |attr|
-        self[attr.to_s].strip!
+        self[attr.to_s].strip! unless self[attr.to_s].nil?
       end
     end
     def strippable_attributes

test/acts_as_stripped_test.rb

@@ -53,6 +53,14 @@ class ActsAsStrippedTest < Test::Unit::TestCase
     setup do
       @post = Post.new
     end
+    should 'not fail if title is nil' do
+      @post.title = nil
+      assert_nothing_raised do
+        @post.save!
+      end
+      assert_nil @post.title
+    end
+
     should 'strip whitespace only from title' do
       @post.title = '  hello world  '
       @post.body = '  awesome  '