Fix GBM & DRF reporting on test sets

Was completely fubar'd.  Was using the class distribution from scoring
the validation set, then passing it thru the model's internal score
code.  For DRF this was mostly a wash; but for GBM you get an extra
Math.exp() pass...
Minor fix for tie-breaking.  Would only pick amongst N-1 choices (never
taking the last choice).  For a 2-way tie, you'd always pick #0.
Happens alot when doing an initial score with zero trees.