SPARK-1791 - SVM implementation does not use threshold parameter

Summary:
https://issues.apache.org/jira/browse/SPARK-1791

Simple fix, and backward compatible, since

- anyone who set the threshold was getting completely wrong answers.
- anyone who did not set the threshold had the default 0.0 value for the threshold anyway.

Test Plan:
Unit test added that is verified to fail under the old implementation,
and pass under the new implementation.

Reviewers:

CC:

Author: Andrew Tulloch <andrew@tullo.ch>

Closes #725 from ajtulloch/SPARK-1791-SVM and squashes the following commits:

770f55d [Andrew Tulloch] SPARK-1791 - SVM implementation does not use threshold parameter

(cherry picked from commit d1e487473fd509f28daf28dcda856f3c2f1194ec)
Signed-off-by: Reynold Xin <rxin@apache.org>