[SPARK-5496][MLLIB] Allow both classification and Classification in Algo for trees.

to be backward compatible.

Author: Xiangrui Meng <meng@databricks.com>

Closes #4287 from mengxr/SPARK-5496 and squashes the following commits:

a025c53 [Xiangrui Meng] Allow both classification and Classification in Algo for trees.

mllib/src/main/scala/org/apache/spark/mllib/tree/configuration/Algo.scala

@@ -29,8 +29,8 @@ object Algo extends Enumeration {
   val Classification, Regression = Value
 
   private[mllib] def fromString(name: String): Algo = name match {
-    case "classification" => Classification
-    case "regression" => Regression
+    case "classification" | "Classification" => Classification
+    case "regression" | "Regression" => Regression
     case _ => throw new IllegalArgumentException(s"Did not recognize Algo name: $name")
   }
 }

mllib/src/test/scala/org/apache/spark/mllib/tree/GradientBoostedTreesSuite.scala

@@ -128,6 +128,11 @@ class GradientBoostedTreesSuite extends FunSuite with MLlibTestSparkContext {
     }
   }
 
+  test("SPARK-5496: BoostingStrategy.defaultParams should recognize Classification") {
+    for (algo <- Seq("classification", "Classification", "regression", "Regression")) {
+      BoostingStrategy.defaultParams(algo)
+    }
+  }
 }
 
 object GradientBoostedTreesSuite {