XML: improve xpath regarding namespaces

[asterite]

Fixes #9279

I checked Nokogiri and if you do this:

require "nokogiri"

str_xml = <<-XML
<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope">
  <soap:Body>
    <NumberToWordsResponse xmlns="http://www.dataaccess.com/webservicesserver/">
      <NumberToWordsResult>string</NumberToWordsResult>
    </NumberToWordsResponse>
  </soap:Body>
</soap:Envelope>
XML

xml = Nokogiri::XML(str_xml)
xml.xpath("/soap:Envelope/soap:Body")

It works out of the box. My guess is that it passes the root namespaces of the document so they can be found with the registered prefixes. So in this PR I do the same.

I also lchop the "xmlns:" prefix of namespaces when registering them. Again, checking what Nokogiri does, the namespaces are returned with the "xmlns:" prefix, but that needs to be removed when registering namespaces for the xpath lookup.