Datamapper DataMapper

Similary to numeric_validator added option :allow_nil to within_validator:

diff --git a/dm-validations/lib/dm-validations/within_validator.rb b/dm-validations/li
index 583363e..018440b 100644
--- a/dm-validations/lib/dm-validations/within_validator.rb
+++ b/dm-validations/lib/dm-validations/within_validator.rb
@@ -14,8 +14,11 @@ module DataMapper
       end
 
       def call(target)
-        includes = @options[:set].include?(target.send(field_name))
-        return true if includes
+        value = target.send(field_name)
+        return true if @options[:allow_nil] && value.nil?
+
+        includes = @options[:set].include?(value)
+        return true if includes || target.send(field_name).nil?
 
         field_name = Extlib::Inflection.humanize(@field_name)
         if @options[:set].is_a?(Range)