python-package: fix eval_at (#639)
python-package: fix eval_at (#639)

eval_at has been broken after e465f92aa389b42da3cd596a2df4a59ae5032de7.
This commit will fix eval_at.
Laurae2/LightGBM,wxchan/LightGBM,Laurae2/LightGBM,wxchan/LightGBM,microsoft/LightGBM,henry0312/LightGBM,olofer/LightGBM,olofer/LightGBM,Allardvm/LightGBM,huanzhang12/LightGBM,wxchan/LightGBM,Allardvm/LightGBM,Laurae2/LightGBM,wxchan/LightGBM,microsoft/LightGBM,microsoft/LightGBM,Allardvm/LightGBM,olofer/LightGBM,henry0312/LightGBM,huanzhang12/LightGBM,Allardvm/LightGBM,henry0312/LightGBM,henry0312/LightGBM,wxchan/LightGBM,Allardvm/LightGBM,olofer/LightGBM,Laurae2/LightGBM,huanzhang12/LightGBM,huanzhang12/LightGBM,microsoft/LightGBM,microsoft/LightGBM,olofer/LightGBM,Laurae2/LightGBM,huanzhang12/LightGBM,henry0312/LightGBM