Fix MiMa

Author: Andrew Or

project/MimaExcludes.scala

@@ -183,7 +183,8 @@ object MimaExcludes {
       ) ++ Seq(
         // SPARK-12896 Send only accumulator updates to driver, not TaskMetrics
         ProblemFilters.exclude[IncompatibleMethTypeProblem]("org.apache.spark.Accumulable.this"),
-        ProblemFilters.exclude[IncompatibleMethTypeProblem]("org.apache.spark.Accumulator.this")
+        ProblemFilters.exclude[IncompatibleMethTypeProblem]("org.apache.spark.Accumulator.this"),
+        ProblemFilters.exclude[MissingMethodProblem]("org.apache.spark.Accumulator.initialValue")
       ) ++ Seq(
         // SPARK-12692 Scala style: Fix the style violation (Space before "," or ":")
         ProblemFilters.exclude[MissingMethodProblem]("org.apache.spark.streaming.flume.sink.SparkSink.org$apache$spark$streaming$flume$sink$Logging$$log_"),