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.

lib/oga/xpath/evaluator.rb

@@ -131,7 +131,8 @@ module Oga
           context = XML::NodeSet.new([@document])
         end
 
-        return on_path(ast_node, context)
+        # If the expression is just "/" we'll just return the current context.
+        return ast_node.children.empty? ? context : on_path(ast_node, context)
       end
 
       ##

lib/oga/xpath/parser.y

@@ -66,6 +66,7 @@ rule
   absolute_path
     : T_SLASH path_members { s(:absolute_path, *val[1]) }
     | T_SLASH path_member  { s(:absolute_path, val[1]) }
+    | T_SLASH              { s(:absolute_path) }
     ;
 
   # Whenever a bare test is used (e.g. just "A") this actually means "child::A".