Number parsing in ROME is currently fairly brittle in many places. While it's
frequently acceptable for numeric properties to be absent, if they are
unparsable a NUmberFormatException is thrown.
It seems like it would be more prudent to treat the situations equivilently.
A specific example seen in the wild (http://feeds.wired.com/GearFactor) is
RSS feeds where some <enclosure> tags have length attributes of "unknown".
patch to follow