[FIX][MLLIB] fix seed handling in Python GMM

If `seed` is `None` on the python side, it will pass in as a `null`. So we should use `java.lang.Long` instead of `Long` to take it.

Author: Xiangrui Meng <[email protected]>

Closes apache#4349 from mengxr/gmm-fix and squashes the following commits:

3be5926 [Xiangrui Meng] fix seed handling in Python GMM

Author: mengxr

mllib/src/main/scala/org/apache/spark/mllib/api/python/PythonMLLibAPI.scala

@@ -295,7 +295,7 @@ class PythonMLLibAPI extends Serializable {
       k: Int, 
       convergenceTol: Double, 
       maxIterations: Int,
-      seed: Long): JList[Object] = {
+      seed: java.lang.Long): JList[Object] = {
     val gmmAlg = new GaussianMixture()
       .setK(k)
       .setConvergenceTol(convergenceTol)