[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/R/xgb.Booster.R

@@ -339,7 +339,7 @@ xgb.attributes <- function(object) {
   # Q: should we warn a user about non-scalar elements?
   a <- lapply(a, function(x) {
     if (is.null(x)) return(NULL)
-    if (is.numeric(value[1])) {
+    if (is.numeric(x[1])) {
       format(x[1], digits = 17)
     } else {
       as.character(x[1])