[R-package] a few fixes for R (#1485)

* [R] fix #1465

* [R] add sanity check to fix #1434

* [R] some clean-ups for custom obj&eval; require maximize only for early stopping

R-package/man/cb.cv.predict.Rd

@@ -34,6 +34,7 @@ Callback function expects the following values to be set in its calling frame:
 \code{basket},
 \code{data},
 \code{end_iteration},
+\code{params},
 \code{num_parallel_tree},
 \code{num_class}.
 }