title: Nested content_tag throws undefined method `output_buffer=` in simple helper
body: <p>I'm trying to create a simple view helper but as soon as I try nest a couple of content tags it will throw <pre>NoMethodError: undefined method `output_buffer=' for</pre></p>
  
  <pre><code>def table_for(list, &amp;proc)
    t = Table.new
    proc.call(t)
    t.render_column(list) 
  end
  
  class Table
    include ActionView::Helpers::TagHelper
  
    attr_accessor :columns, :block
  
    def initialize
      @columns = Array.new
    end
  
    def col(name)
      @columns &lt;&lt; name
    end
  
    def render_column(list)
      content_tag :table do
        list.each do |c|
          content_tag :td, c
        end
      end
    end
  end
  </code></pre>
  
  <p>Any hints of what's wrong? I've also seen that there's a XmlBuilder is that one better for my purpose? </p>