[R] Use built-in label when xgb.DMatrix is given to xgb.cv() (#4631)

* Use built-in label when xgb.DMatrix is given to xgb.cv()

* Add a test

* Fix test

* Bump version number

R-package/tests/testthat/test_callbacks.R

@@ -285,7 +285,8 @@ test_that("prediction in early-stopping xgb.cv works", {
   set.seed(11)
   expect_output(
     cv <- xgb.cv(param, dtrain, nfold = 5, eta = 0.1, nrounds = 20,
-                 early_stopping_rounds = 5, maximize = FALSE, prediction = TRUE)
+                 early_stopping_rounds = 5, maximize = FALSE, stratified = FALSE,
+                 prediction = TRUE)
   , "Stopping. Best iteration")
   
   expect_false(is.null(cv$best_iteration))