Return an ``lxml`` tree as a Unicode string.

def _tree_to_string(cls, root_element, xml_declaration=True, pretty_print=True):
        """
        Return an ``lxml`` tree as a Unicode string.
        """
        from lxml import etree
        return gf.safe_unicode(etree.tostring(
            root_element,
            encoding="UTF-8",
            method="xml",
            xml_declaration=xml_declaration,
            pretty_print=pretty_print
        ))