fixed guts(), clone() and replace_with() to properly handle XML::LibXML::Dtd nodes

guts() now includes the Dtd node in the returned document (unless it were implicitly created)
clone() calls createInternalSubset() on the new document
replace_with() calls createInternalSubset() if the replacement is a XML::LibXML::Dtd (can't import Dtd node)
Checking in changes prior to tagging of version 0.23.

Changelog diff is:

diff --git a/Changes b/Changes
index afbf53e..1ff7987 100644
--- a/Changes
+++ b/Changes
@@ -2,6 +2,14 @@ Revision history for Perl extension HTML::TreeBuilder::LibXML

 {{$NEXT}}

+0.23 2013-05-17T00:16:48Z
+
+    - fixed guts(), clone() and replace_with() to properly handle XML::LibXML::Dtd nodes
+      - guts() now includes the Dtd node in the returned document (unless it were implicitly created)
+      - clone() calls createInternalSubset() on the new document
+      - replace_with() calls createInternalSubset() if the replacement is a XML::LibXML::Dtd (can't import Dtd node)
+        (cafe01)
+
 0.22 2013-05-13T00:04:09Z

     - improved guts(), calling nonBlankChildNodes() instead of childNodes()