Add possibility to not output thead at all

[Marek Hulán]

Sometimes it's good to have a small table without any thead. Right now I can do only something like

table_for connections do |t|
  t.data do
    t.cell(:created_at, :heading => nil
    t.cell(:status, :heading => nil
  end
end

I would prefer to have some option at table_for that would disable tbody generation at all.