Use nested node stacks for predicates.

This ensures the current context node is set correctly when using the "self"
axis inside a path that's inside a predicate, e.g.

    foo/bar[baz/. = "something"]

Here the "self" axis should refer to foo/bar/baz, _not_ foo/bar.

lib/oga/xpath/evaluator.rb

@@ -66,7 +66,7 @@ module Oga
       #
       def initialize(document, variables = {})
         @document  = document
-        @nodes     = []
+        @nodes     = [[]]
         @variables = variables
       end
 
@@ -147,6 +147,7 @@ module Oga
       def on_path(ast_node, context)
         nodes = XML::NodeSet.new
 
+        with_node_stack do
           ast_node.children.each do |test|
             nodes = process(test, context)
 
@@ -156,6 +157,7 @@ module Oga
               context = nodes
             end
           end
+        end
 
         return nodes
       end
@@ -1733,7 +1735,25 @@ module Oga
       # @return [Mixed]
       #
       def with_node(node)
-        @nodes << node
+        node_stack << node
+
+        retval = yield
+
+        node_stack.pop
+
+        return retval
+      end
+
+      ##
+      # Adds a new stack for storing context nodes and yields the supplied
+      # block.
+      #
+      # The return value of this method is whatever the supplied block returns.
+      #
+      # @return [Mixed]
+      #
+      def with_node_stack
+        @nodes << []
 
         retval = yield
 
@@ -1748,6 +1768,13 @@ module Oga
       # @return [Oga::XML::Node]
       #
       def current_node
+        return node_stack.last
+      end
+
+      ##
+      # @return [Array]
+      #
+      def node_stack
         return @nodes.last
       end
     end # Evaluator