[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/callbacks.R

@@ -229,7 +229,7 @@ cb.reset.parameters <- function(new_params) {
       xgb.parameters(env$bst$handle) <- pars
     } else {
       for (fd in env$bst_folds)
-        xgb.parameters(fd$bst$handle) <- pars
+        xgb.parameters(fd$bst) <- pars
     }
   }
   attr(callback, 'is_pre_iteration') <- TRUE