Fix relative URLs when input is stdin.
Fix relative URLs when input is stdin.

marclaporte/WeasyPrint,jasco/WeasyPrint,prepare/TestWeasyPrint,Kozea/WeasyPrint,prepare/TestWeasyPrint,andrewleech/WeasyPrint,jasco/WeasyPrint,Kozea/WeasyPrint,andrewleech/WeasyPrint,marclaporte/WeasyPrint