Don't enable validations when passing false hash values to ActiveModel.validates

Passing a falsey option value for a validator currently causes that validator to
be enabled, just like "true":

    ActiveModel.validates :foo, :presence => false

This is rather counterintuitive, and makes it inconvenient to wrap `validates` in
methods which may conditionally enable different validators.

As an example, one is currently forced to write:

      def has_slug(source_field, options={:unique => true})
        slugger = Proc.new { |r| r[:slug] = self.class.sluggify(r[source_field]) if r[:slug].blank? }
        before_validation slugger
        validations = { :presence => true, :slug => true }
        if options[:unique]
          validations[:uniqueness] = true
        end
        validates :slug, validations
      end

because the following reasonable-looking alternative fails to work as expected:

      def has_slug(source_field, options={:unique => true})
        slugger = Proc.new { |r| r[:slug] = self.class.sluggify(r[source_field]) if r[:slug].blank? }
        before_validation slugger
        validates :slug, :presence => true, :slug => true, :uniqueness => options[:unique]
      end

(This commit includes a test, and all activemodel and activerecord tests pass as before.)

activemodel/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Passing false hash values to `validates` will no longer enable the corresponding validators *Steve Purcell*
+
 *   `ConfirmationValidator` error messages will attach to `:#{attribute}_confirmation` instead of `attribute` *Brian Cardarella*
 
 *   Added ActiveModel::Model, a mixin to make Ruby objects work with AP out of box *Guillermo Iguaran*

activemodel/lib/active_model/validations/validates.rb

@@ -88,6 +88,7 @@ def validates(*attributes)
         defaults.merge!(:attributes => attributes)
 
         validations.each do |key, options|
+          next unless options
           key = "#{key.to_s.camelize}Validator"
 
           begin

activemodel/test/cases/validations_test.rb

@@ -330,6 +330,11 @@ def test_validates_with_bang
     end
   end
 
+  def test_validates_with_false_hash_value
+    Topic.validates :title,  :presence => false
+    assert Topic.new.valid?
+  end
+
   def test_strict_validation_error_message
     Topic.validates :title, :strict => true, :presence => true