code: def header(self, k, v, replace=True):
          
          if replace:
              self._headers[k] = [v]
  
          else:
              self._headers.setdefault(k, []).append(v)
  
          return self
docs: Sets header value. Replaces existing value if `replace` is True.
          Otherwise create a list of existing values and `v`
  
          :param k: Header key
          :param v: Header value
          :param replace: flag for setting mode.
          :type k: str
          :type v: str
          :type replace: bool