Add casts

Author: Feynman Liang

mllib/src/test/scala/org/apache/spark/mllib/linalg/MatricesSuite.scala

@@ -82,11 +82,11 @@ class MatricesSuite extends SparkFunSuite {
     val dm2 = Matrices.dense(2, 2, Array(0.0, 2.0, 1.0, 3.0))
     assert(dm1 === dm2.transpose)
 
-    val sm1 = dm1.toSparse
+    val sm1 = dm1.asInstanceOf[DenseMatrix].toSparse
     assert(sm1 === sm1)
     assert(sm1 !== sm1.transpose)
 
-    val sm2 = dm2.toSparse
+    val sm2 = dm2.asInstanceOf[DenseMatrix].toSparse
     assert(sm1 === sm2.transpose)
   }