XmlRole number() queries should give NaN for non-number values

Given a field like:

<some-number>blah</some-number>

A role like this:

XmlRole { name: "blah"; query: "some-number/number()" }

gives a value of "" when according to the spec (http://www.w3.org/TR/xpath/#section-Number-Functions) it should give a value of "NaN".