Update the PDF's DocumentInfo dictionary to match XMP metadata

        The standard mapping is described here:
            https://www.pdfa.org/pdfa-metadata-xmp-rdf-dublin-core/

def _update_docinfo(self):
        """Update the PDF's DocumentInfo dictionary to match XMP metadata

        The standard mapping is described here:
            https://www.pdfa.org/pdfa-metadata-xmp-rdf-dublin-core/
        """
        self._pdf.docinfo  # Touch object to ensure it exists
        for uri, element, docinfo_name, converter in self.DOCINFO_MAPPING:
            qname = QName(uri, element)
            try:
                value = self[qname]
            except KeyError:
                if docinfo_name in self._pdf.docinfo:
                    del self._pdf.docinfo[docinfo_name]
                continue
            if converter:
                try:
                    value = converter.docinfo_from_xmp(value)
                except ValueError:
                    warn(
                        "The DocumentInfo field {} could not be updated from XMP".format(
                            docinfo_name
                        )
                    )
                    value = None
            if value is None:
                if docinfo_name in self._pdf.docinfo:
                    del self._pdf.docinfo[docinfo_name]
                continue
            value = re_xml_illegal_chars.sub('', value)
            try:
                # Try to save pure ASCII
                self._pdf.docinfo[docinfo_name] = value.encode('ascii')
            except UnicodeEncodeError:
                # qpdf will serialize this as a UTF-16 with BOM string
                self._pdf.docinfo[docinfo_name] = value