add :new_if_blank option to fields_for for nested associations

https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/2365-add-new_if_blank-option-to-fields_for-for-nested-associations

This patch will allow you to initialize any blank associations for your models that are using nested_attributes_for.

# for one-to-one association
form_for(@post) do |f|
   f.text_field(:title)

   f.fields_for(:author, :new_if_blank => true) do |af|
     af.text_field(:name)
   end
end

# for collection associations
# this will create 3 blank comments
form_for(@post) do |f|
   f.text_field(:title)

   f.fields_for(:comments, :new_if_blank => 3) do |cf|
     cf.text_field(:name)
   end
end

Thanks.
Mike