code: def replace_header(self, _name, _value):
          
          _name = _name.lower()
          for i, (k, v) in zip(range(len(self._headers)), self._headers):
              if k.lower() == _name:
                  self._headers[i] = self.policy.header_store_parse(k, _value)
                  break
          else:
              raise KeyError(_name)
docs: Replace a header.
  
          Replace the first matching header found in the message, retaining
          header order and case.  If no matching header was found, a KeyError is
          raised.