[SPARK-12345][MESOS] Properly filter out SPARK_HOME in the Mesos REST server

Fix problem with #10332, this one should fix Cluster mode on Mesos

Author: Iulian Dragos <jaguarul@gmail.com>

Closes #10359 from dragos/issue/fix-spark-12345-one-more-time.

Author: dragos

core/src/main/scala/org/apache/spark/deploy/rest/mesos/MesosRestServer.scala

@@ -99,7 +99,7 @@ private[mesos] class MesosSubmitRequestServlet(
     // cause spark-submit script to look for files in SPARK_HOME instead.
     // We only need the ability to specify where to find spark-submit script
     // which user can user spark.executor.home or spark.home configurations.
-    val environmentVariables = request.environmentVariables.filter(!_.equals("SPARK_HOME"))
+    val environmentVariables = request.environmentVariables.filterKeys(!_.equals("SPARK_HOME"))
     val name = request.sparkProperties.get("spark.app.name").getOrElse(mainClass)
 
     // Construct driver description