[ruby/rexml] Fix attribute's default namespace behavior

NOTE: It's a backward incompatible change. If we have any serious
problems with this change, we may revert this change.

The XML namespace specification says the default namespace doesn't
apply to attribute names but it does in REXML without this change:

https://www.w3.org/TR/xml-names/#uniqAttrs

> the default namespace does not apply to attribute names

REXML reports a parse error for the following XML that is described as
a valid XML in the XML nsmaspace specification without this change:

    <!-- http://www.w3.org is bound to n1 and is the default -->
    <x xmlns:n1="http://www.w3.org"
       xmlns="http://www.w3.org" >
      <good a="1"     b="2" />
      <good a="1"     n1:a="2" />
    </x>

If attribute doesn't have prefix, the attribute should return "" for
both #prefix and #namespace.

https://github.com/ruby/rexml/commit/9e4fd552bc

1 files changed, 49 insertions, 1 deletions

diff --git a/test/rexml/test_core.rb b/test/rexml/test_core.rb
index 46036d7f12..41e6e43540 100644
--- a/test/rexml/test_core.rb
+++ b/test/rexml/test_core.rb
@@ -1,4 +1,4 @@
-# coding: binary
+# -*- coding: utf-8 -*-
 # frozen_string_literal: false
 
 require_relative "rexml_test_utils"
@@ -116,6 +116,54 @@ module REXMLTests
         name4='test4'/>).join(' '), e.to_s
     end
 
+    def test_attribute_namespace_conflict
+      # https://www.w3.org/TR/xml-names/#uniqAttrs
+      message = <<-MESSAGE
+Duplicate attribute "a"
+Line: 4
+Position: 140
+Last 80 unconsumed characters:
+      MESSAGE
+      assert_raise_with_message(REXML::ParseException, message) do
+        Document.new(<<-XML)
+<!-- http://www.w3.org is bound to n1 and n2 -->
+<x xmlns:n1="http://www.w3.org"
+   xmlns:n2="http://www.w3.org" >
+  <bad a="1"     a="2" />
+  <bad n1:a="1"  n2:a="2" />
+</x>
+        XML
+      end
+    end
+
+    def test_attribute_default_namespace
+      # https://www.w3.org/TR/xml-names/#uniqAttrs
+      document = Document.new(<<-XML)
+<!-- http://www.w3.org is bound to n1 and is the default -->
+<x xmlns:n1="http://www.w3.org"
+   xmlns="http://www.w3.org" >
+  <good a="1"     b="2" />
+  <good a="1"     n1:a="2" />
+</x>
+      XML
+      attributes = document.root.elements.collect do |element|
+        element.attributes.each_attribute.collect do |attribute|
+          [attribute.prefix, attribute.namespace, attribute.name]
+        end
+      end
+      assert_equal([
+                     [
+                       ["", "", "a"],
+                       ["", "", "b"],
+                     ],
+                     [
+                       ["", "", "a"],
+                       ["n1", "http://www.w3.org", "a"],
+                     ],
+                   ],
+                   attributes)
+    end
+
     def test_cdata
       test = "The quick brown fox jumped
         & < & < \" '