Fix reverse sort in xpath_parser (#251)

The code below was failing with `REXML::XPathParser#sort': undefined
method '-@' for an instance of Array`
```ruby
d = REXML::Document.new("<a><b><c/><d/><x/></b><b><e/><x/></b></a>")
matches = REXML::XPath.match(d, "a/b/x/preceding-sibling::node()")
# Before: error
# After: [<e/>, <d/>, <c/>]
```
This pull request will fix it.

Author: tompng

lib/rexml/xpath_parser.rb

@@ -671,7 +671,7 @@ def sort(array_of_nodes, order)
         if order == :forward
           index
         else
-          -index
+          index.map(&:-@)
         end
       end
       ordered.collect do |_index, node|

test/xpath/test_base.rb

@@ -416,6 +416,12 @@ def test_preceding
       assert_equal( 4, cs.length )
     end
 
+    def test_preceding_sibling
+      d = REXML::Document.new("<a><b><c/><d/><x/></b><b><e/><x/></b></a>")
+      matches = REXML::XPath.match(d, "a/b/x/preceding-sibling::node()")
+      assert_equal(["e", "d", "c"], matches.map(&:name))
+    end
+
     def test_following
       d = Document.new "<a><b id='0'/><b/><b><c id='1'/><c id='2'/></b><b id='1'/></a>"
       start = XPath.first( d, "/a/b[@id='0']" )