[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, 2 insertions, 2 deletions

diff --git a/lib/rexml/element.rb b/lib/rexml/element.rb
index de83ec3b51..c706a7c245 100644
--- a/lib/rexml/element.rb
+++ b/lib/rexml/element.rb
@@ -1133,8 +1133,8 @@ module REXML
       elsif old_attr.prefix != value.prefix
         # Check for conflicting namespaces
         if value.prefix != "xmlns" and old_attr.prefix != "xmlns"
-          old_namespace = @element.namespace(old_attr.prefix)
-          new_namespace = @element.namespace(value.prefix)
+          old_namespace = old_attr.namespace
+          new_namespace = value.namespace
           if old_namespace == new_namespace
             raise ParseException.new(
                     "Namespace conflict in adding attribute \"#{value.name}\": "+