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.)