Optimize XPath step

If a predicate of xpath is position-independent, we don't need to create nodesets that has many duplicated nodes with different positions.

Author: tompng

lib/rexml/parsers/xpathparser.rb

@@ -654,7 +654,7 @@ def PrimaryExpr path, parsed
           contents = contents[1..-2]
           n = []
           OrExpr( contents, n )
-          parsed.concat(n)
+          parsed.concat([:parentheses, n])
         end
         path
       end