[R-package] JSON dump format and a couple of bugfixes (#1855)

* [R-package] JSON tree dump interface

* [R-package] precision bugfix in xgb.attributes

* [R-package] bugfix for cb.early.stop called from xgb.cv

* [R-package] a bit more clarity on labels checking in xgb.cv

* [R-package] test JSON dump for gblinear as well

* whitespace lint

R-package/tests/testthat/test_callbacks.R

@@ -147,6 +147,11 @@ test_that("cb.reset.parameters works as expected", {
     bst4 <- xgb.train(param, dtrain, nrounds = 2, watchlist,
                       callbacks = list(cb.reset.parameters(my_par)))
   , NA) # NA = no error
+  # CV works as well
+  expect_error(
+    bst4 <- xgb.cv(param, dtrain, nfold = 2, nrounds = 2,
+                   callbacks = list(cb.reset.parameters(my_par)))
+  , NA) # NA = no error
 
   # expect no learning with 0 learning rate
   my_par <- list(eta = c(0., 0.))