REXML::DocType: fix a bug that #clone doesn't copy external ID info

Author: kou

lib/rexml/doctype.rb

@@ -50,6 +50,8 @@ def initialize( first, parent=nil )
         super( parent )
         @name = first.name
         @external_id = first.external_id
+        @long_name = first.instance_variable_get(:@long_name)
+        @uri = first.instance_variable_get(:@uri)
       elsif first.kind_of? Array
         super( parent )
         @name = first[0]