Set the row's header from a list.

def _SetHeader(self, values):
    """Set the row's header from a list."""
    if self._values and len(values) != len(self._values):
      raise ValueError('Header values not equal to existing data width.')
    if not self._values:
      for _ in range(len(values)):
        self._values.append(None)
    self._keys = list(values)
    self._BuildIndex()