SPARK-1428: MLlib should convert non-float64 NumPy arrays to float64 instead of complaining

Author: Sandeep <sandeep@techaddict.me>

Closes #356 from techaddict/1428 and squashes the following commits:

3bdf5f6 [Sandeep] SPARK-1428: MLlib should convert non-float64 NumPy arrays to float64 instead of complaining

(cherry picked from commit 3bd312940e2f5250edaf3e88d6c23de25bb1d0a9)
Signed-off-by: Matei Zaharia <matei@databricks.com>