Fix OOB score calculation for non-contiguous targets

OOB scores of RandomForestClassifier are incorrectly calculated
because of confusion between predicted labels and indices of a target.
For example, when you label the `digits` dataset from 1 or greater
number but 0, OOB score becomes nearly zero.
This patch will fix this error, and a test is added in order to avoid
regression.
Merge pull request #2685 from bicycle1885/fix-oob-score

Fix OOB score calculation for non-contiguous targets