added test for training when both train and valid are subsets of a si… (#759)
added test for training when both train and valid are subsets of a si… (#759)

* added test for training when both train and valid are subsets of a single lgb.Dataset object

* pep8 changes

* more pep8

* added test involving subsets of subsets of lgb.Dataset objects

* minor fix to contruction of X matrix

* even more pep8

* simplified test further

henry0312/LightGBM,Allardvm/LightGBM,Laurae2/LightGBM,Allardvm/LightGBM,wxchan/LightGBM,Allardvm/LightGBM,henry0312/LightGBM,microsoft/LightGBM,olofer/LightGBM,wxchan/LightGBM,Allardvm/LightGBM,microsoft/LightGBM,microsoft/LightGBM,olofer/LightGBM,henry0312/LightGBM,Laurae2/LightGBM,Laurae2/LightGBM,olofer/LightGBM,Laurae2/LightGBM,olofer/LightGBM,Laurae2/LightGBM,wxchan/LightGBM,wxchan/LightGBM,microsoft/LightGBM,Allardvm/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,olofer/LightGBM,wxchan/LightGBM