Update wsfedsaml.js

The 'xpath' function appears to be an alias for SelectNodes, so you can't call 'SelectNodes.SelectNodes'

in the documentation for xml-crypto, 'xpath' is used instead of 'SelectNodes', so making this match.
Merge pull request #3 from desirable-objects/master

Update wsfedsaml.js as previously it was throwing an error and would not allow login.