XHTML-Print, 3.8 basic table, default align of th should be cente red

The XHTML-Print CR spec says in section 3.8 [1], "If the align attribute is
missing or has an unsupported value a printer MUST act as if the align
attribute has the value left." 

This should only be for tr and td, the default align for th should be
center.

Jim
[1] http://www.w3.org/TR/2004/CR-xhtml-print-20040120/#s3.8