xpath: Fix wrong position with nested path (#122)

## Why?
Fixed incorrect calculation of position in node set.

Fix GH-25

Reported by jcavalieri. Thanks!!!

lib/rexml/xpath_parser.rb

@@ -590,6 +590,7 @@ def filter_nodeset(nodeset)
 
     def evaluate_predicate(expression, nodesets)
       enter(:predicate, expression, nodesets) if @debug
+      new_nodeset_count = 0
       new_nodesets = nodesets.collect do |nodeset|
         new_nodeset = []
         subcontext = { :size => nodeset.size }
@@ -606,17 +607,20 @@ def evaluate_predicate(expression, nodesets)
           result = result[0] if result.kind_of? Array and result.length == 1
           if result.kind_of? Numeric
             if result == node.position
-              new_nodeset << XPathNode.new(node, position: new_nodeset.size + 1)
+              new_nodeset_count += 1
+              new_nodeset << XPathNode.new(node, position: new_nodeset_count)
             end
           elsif result.instance_of? Array
             if result.size > 0 and result.inject(false) {|k,s| s or k}
               if result.size > 0
-                new_nodeset << XPathNode.new(node, position: new_nodeset.size + 1)
+                new_nodeset_count += 1
+                new_nodeset << XPathNode.new(node, position: new_nodeset_count)
               end
             end
           else
             if result
-              new_nodeset << XPathNode.new(node, position: new_nodeset.size + 1)
+              new_nodeset_count += 1
+              new_nodeset << XPathNode.new(node, position: new_nodeset_count)
             end
           end
         end

test/xpath/test_base.rb

@@ -451,6 +451,46 @@ def test_following
     #  puts results
     #end
 
+    def test_nested_predicates
+      doc = Document.new <<-EOF
+        <div>
+          <div>
+            <test>ab</test>
+            <test>cd</test>
+          </div>
+          <div>
+            <test>ef</test>
+            <test>gh</test>
+          </div>
+          <div>
+            <test>hi</test>
+          </div>
+        </div>
+      EOF
+
+      matches = XPath.match(doc, '(/div/div/test[0])').map(&:text)
+      assert_equal [], matches
+      matches = XPath.match(doc, '(/div/div/test[1])').map(&:text)
+      assert_equal ["ab", "ef", "hi"], matches
+      matches = XPath.match(doc, '(/div/div/test[2])').map(&:text)
+      assert_equal ["cd", "gh"], matches
+      matches = XPath.match(doc, '(/div/div/test[3])').map(&:text)
+      assert_equal [], matches
+
+      matches = XPath.match(doc, '(/div/div/test[1])[1]').map(&:text)
+      assert_equal ["ab"], matches
+      matches = XPath.match(doc, '(/div/div/test[1])[2]').map(&:text)
+      assert_equal ["ef"], matches
+      matches = XPath.match(doc, '(/div/div/test[1])[3]').map(&:text)
+      assert_equal ["hi"], matches
+      matches = XPath.match(doc, '(/div/div/test[2])[1]').map(&:text)
+      assert_equal ["cd"], matches
+      matches = XPath.match(doc, '(/div/div/test[2])[2]').map(&:text)
+      assert_equal ["gh"], matches
+      matches = XPath.match(doc, '(/div/div/test[2])[3]').map(&:text)
+      assert_equal [], matches
+    end
+
     # Contributed by Mike Stok
     def test_starts_with
       source = <<-EOF