rexml: Fix XPath bug of /#{ELEMENT_NAME}

It doesn't mean that all elements which name "ELEMENT_NAME" with any
namespace URI including null namespace URI. It means that all elements
which name "ELEMENT_NAME" with null namespace URI.

https://www.w3.org/TR/1999/REC-xpath-19991116/#NT-NodeTest

> if the QName does not have a prefix, then the namespace URI is null
> (this is the same way attribute names are expanded).

We need to use "*[local-name()='#{ELEMENT_NAME}']" for all elements
which name "ELEMENT_NAME" with any namespace URI including null
namespace URI in XPath 1.0. But it's inconvenient. So this change
includes "*:#{LOCAL_NAME}" syntax support that is introduced since
XPath 2.0.

* lib/rexml/parsers/xpathparser.rb: Support "*:#{LOCAL_NAME}" syntax that
  is introduced since XPath 2.0.

* lib/rexml/xpath_parser.rb:
  * Fix namespace URI processing for "#{ELEMENT_NAME}". Now,
    "#{ELEMENT_NAME}" doesn't accept elements with null namespace URI.
  * Add "*:#{LOCAL_NAME}" support.

* test/rexml/test_contrib.rb,
  test/rexml/test_core.rb,
  test/rexml/xpath/test_base.rb: Follow this change.

* test/rexml/test_jaxen.rb: Fix namespace processing.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63236 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/rexml/test_contrib.rb

@@ -451,7 +451,7 @@ EOL
     end
 
     def test_external_entity
-      xp = '//channel/title'
+      xp = '//*:channel/*:title'
 
       %w{working.rss broken.rss}.each do |path|
         File.open(File.join(fixture_path(path))) do |file|