code: def header_output(self):
          '''只输出cookie的key-value字串.
          
          比如: HISTORY=21341; PHPSESSION=3289012u39jsdijf28; token=233129
          '''
          result = []
          for key in self.keys():
              result.append(key + '=' + self.get(key).value)
          return '; '.join(resultf header_output(self):
          '''只输出cookie的key-value字串.
          
          比如: HISTORY=21341; PHPSESSION=3289012u39jsdijf28; token=233129
          '''
          result = []
          for key in self.keys():
              result.append(key + '=' + self.get(key).value)
          return '; '.join(result)
docs: 只输出cookie的key-value字串.
          
          比如: HISTORY=21341; PHPSESSION=3289012u39jsdijf28; token=233129