CiviReport: Layout problem multiple-page PDF

I ran the report 'relationship' (which can be found under contact reports) to show all relationships of a certain type. I exported the results to a pdf, we use wkhtmltopdf 0.12.3 (with patched qt).

This generated a multi-page pdf, with a layout problem. On pages 2 and higher, the table header is printed 'above' the report data, as shown in the attached image.

This seems to be an issue of wkhtmltopdf, and can be fixed by adding 3 lines of css: https://github.com/wkhtmltopdf/wkhtmltopdf/issues/1524#issuecomment-85072480.

I will create a PR in a moment.