Code spans in table heading not supported

[richanna]

Surrounding a table heading in back ticks results in a non-fatal error message, and an empty table heading.

e.g., The table heading: | ``foo`` | bar

Produces the error: *** lost markup <spanx style="verb">foo</spanx> in table heading

And renders equivalently to: | | bar

[cabo]

Right, that was a limitation of RFCXMLv2. We can fix that by outputting tables in RFCXMLv3.