content_tag_for should behave like content_tag

The attached patch allows content_tag_for to be used in all cases where content_tag should work.

As it stands calling content_tag_for in a helper, with a block ala...

def voodoo_summary(voodoo)
  div_for(voodoo) do
    h(voodoo.name) + " has " +
    pluralise(voodoo.spells.count, "spells") +
    link_to("[more details on #{voodoo.name}]", voodoo)
  end
end

When this is called from the erb template...

<% @spell_book.voodoo.each do |voodoo| %>
  <%= voodoo_summary(voodoo) %>
<% end %>

We end up getting multiple renders and worse.

Attached patch makes content_tag_for use content_tag in a DRYer way... using it's code to prevent this.

Test included for good luck.