Zero-arity conditional validation procs broken by AS::NewCallbacks

Using a zero-arity proc/lambda as an :if or :unless option for validations, breaks with the new callback code. This potentially affects a lot of legacy code.

Consider the following code:

class User
  include ActiveModel::Validations
  attr_accessor :name
  validates_presence_of :name, :if => Proc.new { true }
end

>> User.new.valid?
ArgumentError: tried to create Proc object without a block
    from /Users/sam/new-rails-2/vendor/rails/activeresource/lib/../../activesupport/lib/active_support/new_callbacks.rb:396:in `new'
    from /Users/sam/new-rails-2/vendor/rails/activeresource/lib/../../activesupport/lib/active_support/new_callbacks.rb:396:in `_run_validate_callbacks'
    from /Users/sam/new-rails-2/vendor/rails/activeresource/lib/../../activemodel/lib/active_model/validations.rb:94:in `valid?'
    from (irb):2

In AS::NewCallbacks::_compile_filter, replacing

method_name << (filter.arity == 1 ? "(self)" : " self, Proc.new ")

with:

method_name << (filter.arity == 1 ? "(self)" : " self, Proc.new {} ")

appears to fix the problem and passes all tests, but I'm not sure what exactly the second proc argument does there.

The attached patch includes failing tests for ActiveModel.