This is useful in tests because the doctest comparison engine from lxml
works better with text objects. The new _encoding argument is
essentially used a "encoding" argument of lxml's etree.tostring() which
will hence serialize to a unicode string. When doing so, we must also
avoid any XML declaration (otherwise, lxml messes up when loading the
content, not sure why), so we insert the "standalone" flag only when
dumping to bytes.