Build fix.

        * xml/XPathUtil.cpp:
        (WebCore::XPath::stringValue):

LayoutTests:
        Added a test for attribute node string-value.

        * fast/xpath/string-value-expected.txt:
        * fast/xpath/string-value.html:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@20057 268f45cc-cd09-0410-ab3c-d52691b4dbfc