Allow for namespaces declared outside of the root element in single sign out requests

[chuckbjones]

The CAS server I'm using declares the saml namespace inside the NameID element:

<samlp:LogoutRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" ID="foobarbaz" Version="2.0" IssueInstant="2014-10-19T17:13:50Z">
    <saml:NameID xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">@NOT_USED@</saml:NameID>
    <samlp:SessionIndex>ST-foo-bar</samlp:SessionIndex>
</samlp:LogoutRequest>

Nokogiri only looks for namespace declarations in the root element, then raises an exception when it can't find one that was declared elsewhere. This PR forces Nokogiri to scan the entire xml document for namespaces before accessing any nodes.

[nicolasfranck]

I was looking for the same thing!
Is there still working on this project?

[dlindahl]

I no longer have access to a CAS server and therefore am no longer fit to maintain this project.

If you would like to volunteer to be a maintainer of this project, please let me know by opening an Issue.

[vjt]

Superseded by #39