LightGBM

[R-package] Turn matrix to storage mode "double" (#3140)

* Turn matrix to storage mode "double"

* added "test_Predictor.R" to R tests with check for integer storage mode during prediction

* Explicit integers in test_Preditor to avoid TravisCI failure

* properly aligning comment on storage.mode

Co-authored-by: James Lamb <jaylamb20@gmail.com>

* split double assignment into two lines

Co-authored-by: James Lamb <jaylamb20@gmail.com>