Added failing descendant-or-self XPath specs.

These currently fail due to the child:: selector not working entirely as it
should be. Consider the following XML:

    <a><b><b><c class="x"></c></b></b></a>

And the following XPath:

    descendant-or-self::node()/a

In Nokogiri/libxml this will return a node set containing the <a> node. In Oga
however this will return an empty node set. This will require some further
investigation to see what exactly is going on, and in particular what is the
correct behaviour.

spec/oga/xpath/evaluator/axes/descendant_or_self_spec.rb

@@ -40,6 +40,32 @@ describe Oga::XPath::Evaluator do
       end
     end
 
+    context 'chained descendants' do
+      before do
+        @set = @evaluator.evaluate(
+          'descendant-or-self::a/descendant-or-self::*'
+        )
+      end
+
+      it_behaves_like :node_set, :length => 4
+
+      example 'return the <a> node' do
+        @set[0].should == @first_a
+      end
+
+      example 'return the first <b> node' do
+        @set[1].should == @first_b
+      end
+
+      example 'return the second <b> node' do
+        @set[2].should == @second_b
+      end
+
+      example 'return the <c> node' do
+        @set[3].should == @first_c
+      end
+    end
+
     context 'nested descendants with a matching predicate' do
       before do
         @set = @evaluator.evaluate('descendant-or-self::c[@class="x"]')
@@ -139,5 +165,37 @@ describe Oga::XPath::Evaluator do
         @set[0].should == @first_c
       end
     end
+
+    context 'descendants of descendants usign the short form' do
+      before do
+        @set = @evaluator.evaluate('//a//*')
+      end
+
+      it_behaves_like :node_set, :length => 3
+
+      example 'return the first <b> node' do
+        @set[0].should == @first_b
+      end
+
+      example 'return the second <b> node' do
+        @set[1].should == @second_b
+      end
+
+      example 'return the <c> node' do
+        @set[2].should == @first_c
+      end
+    end
+
+    context 'children of descendants using the short form' do
+      before do
+        @set = @evaluator.evaluate('//a/b')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the first <b> node' do
+        @set[0].should == @first_b
+      end
+    end
   end
 end