Emit with toHexString() instead of toString() since R 3.1 parsing became
far less tolerant.
PUB-571: Changes for R 3.1 support.

Added hex_string option to DownloadDataset.  This has java emit a
perfect lossless representation of the double value.  R is able to
read this with read.csv().

as.data.frame specifies hex_string.