xpah abbreviate: add missing "/" to :descendant_or_self/:self/:parent

GitHub: fix GH-97

Reported by pulver. Thanks!!!

Author: kou

lib/rexml/parsers/xpathparser.rb

@@ -52,11 +52,11 @@ def abbreviate( path )
           when :child
             string << "/" if string.size > 0
           when :descendant_or_self
-            string << "/"
+            string << "//"
           when :self
-            string << "."
+            string << "/"
           when :parent
-            string << ".."
+            string << "/.."
           when :any
             string << "*"
           when :text