[jvm-packages] Objectives starting with rank: are never classification (#2837)

Training a model with the experimental rank:ndcg objective incorrectly
returns a Classification model. Adjust the classification check to
not recognize rank:* objectives as classification.

While writing tests for isClassificationTask also turned up that
obj_type -> regression was incorrectly identified as a classification
task so the function was slightly adjusted to pass the new tests.

jvm-packages/xgboost4j-spark/src/main/scala/ml/dmlc/xgboost4j/scala/spark/XGBoost.scala

@@ -194,8 +194,8 @@ object XGBoost extends Serializable {
     val objective = params.getOrElse("objective", params.getOrElse("obj_type", null))
     objective != null && {
       val objStr = objective.toString
-      objStr == "classification" || (!objStr.startsWith("reg:") && objStr != "count:poisson" &&
-        objStr != "rank:pairwise")
+      objStr != "regression" && !objStr.startsWith("reg:") && objStr != "count:poisson" &&
+        !objStr.startsWith("rank:")
     }
   }