[SPARK-18678][ML] Skewed reservoir sampling in SamplingUtils

## What changes were proposed in this pull request?

Fix reservoir sampling bias for small k. An off-by-one error meant that the probability of replacement was slightly too high -- k/(l-1) after l element instead of k/l, which matters for small k.

## How was this patch tested?

Existing test plus new test case.

Author: Sean Owen <sowen@cloudera.com>

Closes #16129 from srowen/SPARK-18678.

(cherry picked from commit 79f5f281)
Signed-off-by: Sean Owen <sowen@cloudera.com>