Unfuck XPath axes evaluation.

The evaluation of axes has been fixed by changing the initial context as well as
the behaviour of some of the handler methods.

The initial context has been changed so that it's simply a NodeSet of whatever
the root object is, either a Document or an Element instance. Previously this
would be set to the child nodes of a Document in case the root object was a
Document. This in turn would prevent "child" axes from operating correctly.

lib/oga/xpath/evaluator.rb

@@ -25,12 +25,7 @@ module Oga
       #
       def evaluate(string)
         ast     = Parser.new(string).parse
-
-        if @document.is_a?(XML::Document)
-          context = @document.children
-        else
         context = XML::NodeSet.new([@document])
-        end
 
         return process(ast, context)
       end
@@ -79,16 +74,15 @@ module Oga
       # @return [Oga::XML::NodeSet]
       #
       def on_path(node, context)
-        last_node = node.children[-1]
         nodes = XML::NodeSet.new
 
         node.children.each do |test|
           nodes = process(test, context)
 
-          if test != last_node and !nodes.empty?
-            context = child_nodes(context)
-          elsif nodes.empty?
+          if nodes.empty?
             break
+          else
+            context = nodes
           end
         end
 
@@ -214,7 +208,7 @@ module Oga
       # @return [Oga::XML::NodeSet]
       #
       def on_axis_child(node, context)
-        return on_test(node, context)
+        return on_test(node, child_nodes(context))
       end
 
       ##
@@ -227,10 +221,10 @@ module Oga
       #
       def on_axis_descendant(node, context)
         nodes = XML::NodeSet.new
-        children = child_nodes(context)
 
-        unless children.empty?
-          nodes += on_axis_descendant(node, children)
+        context.each do |context_node|
+          nodes += on_test(node, context_node.children)
+          nodes += on_axis_descendant(node, context_node.children)
         end
 
         return nodes
@@ -244,14 +238,7 @@ module Oga
       # @return [Oga::XML::NodeSet]
       #
       def on_axis_descendant_or_self(node, context)
-        nodes    = on_test(node, context)
-        children = child_nodes(context)
-
-        unless children.empty?
-          nodes += on_axis_descendant(node, children)
-        end
-
-        return nodes
+        return on_test(node, context) + on_axis_descendant(node, context)
       end
 
       ##
@@ -265,9 +252,7 @@ module Oga
         children = XML::NodeSet.new
 
         nodes.each do |xml_node|
-          xml_node.children.each do |child|
-            children << child
-          end
+          children += xml_node.children
         end
 
         return children