[SPARK-2515][mllib] Chi Squared test

the following style may be better:

~~~
def chiSqTest(observed: Vector, expected: Vector): ChiSquaredTestResul =
  ChiSquaredTest.chiSquared(observed, expected)
~~~

~~~
def chiSqTest(observed: Vector, expected: Vector): ChiSquaredTestResult = {
  ChiSquaredTest.chiSquared(observed, expected)
}
~~~