text: Title: Using GaussianProcessRegressor fails for default values
  Body: ```python
  from skopt.learning import GaussianProcessRegressor
  
  dimensions = [(-5.0, 10.0), (0.0, 15.0)]
  gpr = GaussianProcessRegressor()
  optimizer = Optimizer(
      dimensions=dimensions,
      base_estimator=gpr,
      n_random_starts=1,
      acq_func="EI",
      random_state=0)
  
  optimizer.tell(optimizer.ask(), branin(x_cand))
  
  AttributeError: 'Product' object has no attribute 'gradient_x'
  ```