docs: def xml_filter(self, content):
          r"""Filter and preprocess xml content
  
          :param content: xml content
          :rtype: str
          """
          content = utils.strip_whitespace(content, True) if self.__options['strip'] else content.strip()
  
          if not self.__options['encoding']:
              encoding = self.guess_xml_encoding(content) or self.__encoding
              self.set_options(encoding=encoding)
  
          if self.__options['encoding'].lower() != self.__encoding:
              # 编码转换去除xml头
              content = self.strip_xml_header(content.decode(self.__options['encoding'], errors=self.__options['errors']))
  
          if self.__options['unescape']:
              content = utils.html_entity_decode(content)
          return content