Small tweaks to provide more info about generated prediction from H2O.
Permit empty domain in transforming into enum.

toEnum call can be call on a vector which does not contains any domain
values, only NA. To preserve toEnum contract (every call returns a new
vector), we permit this situation.
Fix in shared tree builder. The last tree was not saved.
Merge branch 'master' of github.com:0xdata/h2o

Conflicts:
	R/tests/Utils/shared_javapredict_RF.R