Support for using namespace-uri() with attributes.

spec/oga/xpath/evaluator/calls/namespace_uri_spec.rb

@@ -3,7 +3,7 @@ require 'spec_helper'
 describe Oga::XPath::Evaluator do
   context 'namespace-uri() function' do
     before do
-      @document  = parse('<root xmlns:x="y"><x:a></x:a><b></b></root>')
+      @document  = parse('<root xmlns:x="y"><x:a></x:a><b x:num="10"></b></root>')
       @evaluator = described_class.new(@document)
     end
 
@@ -12,6 +12,10 @@ describe Oga::XPath::Evaluator do
         @evaluator.evaluate('namespace-uri(root/x:a)').should == 'y'
       end
 
+      example 'return the namespace URI of the "num" attribute' do
+        @evaluator.evaluate('namespace-uri(root/b/@x:num)').should == 'y'
+      end
+
       example 'return an empty string when there is no namespace URI' do
         @evaluator.evaluate('namespace-uri(root/b)').should == ''
       end