fix convertHashAggregateExec ReflectionException (#637)

WARN BlazeConverters: Error converting exec: HashAggregateExec: expected a member of class HashAggregateExec, you provided value org.apache.spark.sql.execution.aggregate.HashAggregateExec.requiredChildDistributionExpressions
scala.ScalaReflectionException: expected a member of class HashAggregateExec, you provided value org.apache.spark.sql.execution.aggregate.HashAggregateExec.requiredChildDistributionExpressions
        at scala.reflect.runtime.JavaMirrors$JavaMirror.scala$reflect$runtime$JavaMirrors$JavaMirror$$abort(JavaMirrors.scala:155)
        at scala.reflect.runtime.JavaMirrors$JavaMirror.ErrorNotMember(JavaMirrors.scala:161)
        at scala.reflect.runtime.JavaMirrors$JavaMirror.$anonfun$checkMemberOf$1(JavaMirrors.scala:256)
        at scala.reflect.runtime.JavaMirrors$JavaMirror.scala$reflect$runtime$JavaMirrors$JavaMirror$$checkMemberOf(JavaMirrors.scala:246)
        at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaInstanceMirror.reflectField(JavaMirrors.scala:278)
        at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaInstanceMirror.reflectField(JavaMirrors.scala:275)
        at org.apache.spark.sql.blaze.BlazeConverters$.$anonfun$convertHashAggregateExec$1(BlazeConverters.scala:616)
        at scala.collection.immutable.List.map(List.scala:293)
        at org.apache.spark.sql.blaze.BlazeConverters$.convertHashAggregateExec(BlazeConverters.scala:611)
        at org.apache.spark.sql.blaze.BlazeConverters$.$anonfun$convertSparkPlan$15(BlazeConverters.scala:177)
        at org.apache.spark.sql.blaze.BlazeConverters$.tryConvert(BlazeConverters.scala:243)

spark-extension/src/main/scala/org/apache/spark/sql/blaze/BlazeConverters.scala

@@ -615,7 +615,7 @@ object BlazeConverters extends Logging {
                   case "aggregateExpressions" => transformedAggregateExprs
                   case "groupingExpressions" => transformedGroupingExprs
                   case "child" => convertProjectExec(ProjectExec(projections, exec.child))
-                  case _ => mirror.reflectField(param.asTerm).get
+                  case _ => mirror.reflectField(typeOf[HashAggregateExec].decl(TermName(param.name.toString)).asTerm).get
                 }
               }
               mirror.reflectMethod(copyMethod)(args: _*).asInstanceOf[HashAggregateExec]