code: def get(self, name: str) -> Union[None, str, List[str]]:
          
          name = name.casefold()
          if name == "referer" or name == "referrer":
              if "referrer" in self._headers:
                  return self._headers["referrer"]
              elif "referer" in self._headers:
                  return self._headers["referer"]
              else:
                  return None
          elif name in self._headers:
              return self._headers[name]
          else:
              return None
docs: 获取 header