Fixed ancestor-or-self relative to attributes

Per libxml behaviour this axis shouldn't match attributes when using
"ancestor-or-self::*".

lib/oga/xpath/compiler.rb

@@ -176,7 +176,7 @@ module Oga
       def on_axis_ancestor_or_self(ast, input)
         parent = unique_literal(:parent)
 
-        process(ast, input)
+        process(ast, input).and(input.is_a?(XML::Node))
           .if_true { yield input }
           .followed_by do
             attribute_or_node(input).if_true do

spec/oga/xpath/compiler/axes/ancestor_or_self_spec.rb

@@ -23,6 +23,12 @@ describe Oga::XPath::Compiler do
         evaluate_xpath(@a1.attribute('foo')).should == node_set(@a1)
       end
     end
+
+    describe 'ancestor-or-self::foo' do
+      it 'returns an empty NodeSet' do
+        evaluate_xpath(@a1.attribute('foo')).should == node_set
+      end
+    end
   end
 
   describe 'relative to an element' do