Nested fixed elements produce error on multi-page layouts

[iabw]

When content is only 1 page in length, nested fixed position elements work as I expect. When content is more than 1 page in length, Weasyprint throws a TypeError.

Error Produced:
TypeError: coercing to Unicode: need string or buffer, int found

Reduced test case:

<style>
    .header{ position: fixed; top: 0 }
    .footer { position: fixed; bottom: 0; }
</style>

<div>Text</div> <!-- Repeated enough to produce more than 1 page of content -->

<div class='header'>
    <div class='footer'>Footer</div>
</div>

Traceback:

  File "/weasyprint/__init__.py", line 132, in render
    return Document._render(self, stylesheets, enable_hinting)
  File "/weasyprint/document.py", line 308, in _render
    return cls([Page(p, enable_hinting) for p in page_boxes],
  File "/weasyprint/layout/__init__.py", line 62, in layout_document
    layout_backgrounds(page, get_image_from_uri)
  File "/weasyprint/layout/backgrounds.py", line 213, in layout_backgrounds
    layout_box_backgrounds(page, page, get_image_from_uri)
  File "/weasyprint/layout/backgrounds.py", line 59, in layout_box_backgrounds
    layout_box_backgrounds(page, child, get_image_from_uri)
  File "/weasyprint/layout/backgrounds.py", line 59, in layout_box_backgrounds
    layout_box_backgrounds(page, child, get_image_from_uri)
  File "/weasyprint/layout/backgrounds.py", line 59, in layout_box_backgrounds
    layout_box_backgrounds(page, child, get_image_from_uri)
  File "/weasyprint/layout/backgrounds.py", line 56, in layout_box_backgrounds
    resolve_radiii_percentages(box)
  File "/weasyprint/layout/percentages.py", line 125, in resolve_radiii_percentages
    ry = _percentage(ry, box.border_height())
  File "/weasyprint/formatting_structure/boxes.py", line 151, in border_height
    return self.padding_height() + self.border_top_width + \
  File "/weasyprint/formatting_structure/boxes.py", line 142, in padding_height
    return self.height + self.padding_top + self.padding_bottom
TypeError: coercing to Unicode: need string or buffer, int found

[liZe]

Fixed with 1eea5a4 (see #728).