code: def handel_default(self) -> None:
          
          raw_body = self._body
          body = cast(Optional[bytes], None)
          default_type = 2
          charset = self._charset or self._default_charset
          if raw_body is None:
              pass
          elif isinstance(raw_body, bytes):
              # body为bytes
              default_type = 2
              body = raw_body
          elif isinstance(raw_body, str):
              # body 为字符串
              default_type = 2
              body = encode_str(raw_body, charset)
          elif isinstance(raw_body, (list, dict)):
              # body 为json
              default_type = 3
              body = encode_str(json.dumps(raw_body, ensure_ascii=False), charset)
          elif isinstance(raw_body, RawIOBase):
              # body 为文件
              default_type = 1
              body = raw_body.read()
              raw_body.close()
          if "Content-Length" not in self._headers and \
                  "Transfer-Encoding" not in self._headers \
                  or self._headers["Transfer-Encoding"] != "chunked":
              if self.length is None:
                  if body is not None:
                      self.length = len(body)
                  else:
                      self.length = 0
              # 设置默认 Content-Length
              self.set("Content-Length", str(self.length))
          # print(body[0], body[1])
          if body is not None and body.startswith(encode_str("<", charset)):
              default_type = 4
          if "Content-Type" not in self._headers.keys():
              type_str = self.type
              if type_str is None:
                  temp = DEFAULT_TYPE.get(default_type)
                  if temp is not None:
                      if default_type != 1:
                          temp += "; charset=%s" % charset
                      type_str = temp
              if type_str is not None:
                  # 设置默认 Content-Type
                  self.set("Content-Type", type_str)
          self._body = body
docs: 处理设置到body上的数据默认 headers