XPath support for absolute paths without tests.

This allows Oga to parse and evaluate the XPath expression "/". This expression
can be used to select just the root node/document.

This fixes #35.

spec/oga/xpath/evaluator/paths_spec.rb

@@ -30,6 +30,18 @@ describe Oga::XPath::Evaluator do
     end
   end
 
+  context 'absolute paths without node tests' do
+    before do
+      @set = described_class.new(@document).evaluate('/')
+    end
+
+    it_behaves_like :node_set, :length => 1
+
+    example 'return the root document' do
+      @set[0].is_a?(Oga::XML::Document).should == true
+    end
+  end
+
   context 'invalid absolute paths' do
     before do
       @set = described_class.new(@document).evaluate('/x/a')

spec/oga/xpath/parser/paths_spec.rb

@@ -20,5 +20,9 @@ describe Oga::XPath::Parser do
         s(:axis, 'child', s(:test, nil, 'B'))
       )
     end
+
+    example 'parse an absolute path without a node test' do
+      parse_xpath('/').should == s(:absolute_path)
+    end
   end
 end