HTML::TreeBuilder and XPath compatible interface with libxml

Changes for 0.27 - 2024-01-09T06:19:12Z

• don't have the parser string blanks by default #15 https://github.com/tokuhirom/HTML-TreeBuilder-LibXML/pull/15
• fallback to toString() when toStringC14N(1) gives an empty string #17 https://github.com/tokuhirom/HTML-TreeBuilder-LibXML/pull/17