Add initial x coordinate to add_header

Hi,

This is a new pull request to replace #372. Here is an example that produces the problem:

```ruby
# This file is bugs/column_boxes_and_tables.rb
$LOAD_PATH << File.join(File.dirname(__FILE__), '..', 'lib')
require "prawn"
Prawn::Document.generate('headers_drawing_on_same_column.pdf') do
  data = [["This row should be repeated on every new page AND COLUMN"]]
  data += [["..."]] * 50
  column_box([0, cursor], :columns => 2, :width => 
bounds.width) do
    table(data, :header => true)
  end
end
```
This produces: 
![headers_drawing_on_same_column pdf__page_1_of_2_-7](https://f.cloud.github.com/assets/4662860/1568277/33f5b828-50ac-11e3-9260-26f886aeb253.jpg)

Thanks for your time and let me know what you think!