docs: def add_header(self, header):
          """Add headers to the email globaly or to a specific Personalization
  
          :param value: A Header object or a dict of header key/values
          :type value: Header, dict
          """
          if header.personalization is not None:
              try:
                  personalization = \
                      self._personalizations[header.personalization]
                  has_internal_personalization = True
              except IndexError:
                  personalization = Personalization()
                  has_internal_personalization = False
              if isinstance(header, dict):
                  (k, v) = list(header.items())[0]
                  personalization.add_header(Header(k, v))
              else:
                  personalization.add_header(header)
  
              if not has_internal_personalization:
                  self.add_personalization(
                      personalization,
                      index=header.personalization)
          else:
              if isinstance(header, dict):
                  (k, v) = list(header.items())[0]
                  self._headers = self._ensure_append(
                                      Header(k, v), self._headers)
              else:
                  self._headers = self._ensure_append(header, self._headers)