[SPARK-17369][SQL] MetastoreRelation toJSON throws AssertException due to missing otherCopyArgs

## What changes were proposed in this pull request?

`TreeNode.toJSON` requires a subclass to explicitly override otherCopyArgs to include currying construction arguments, otherwise it reports AssertException telling that the construction argument values' count doesn't match the construction argument names' count.

For class `MetastoreRelation`, it has a currying construction parameter `client: HiveClient`, but Spark forgets to add it to the list of otherCopyArgs.

## How was this patch tested?

Unit tests.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #14928 from clockfly/metastore_relation_toJSON.

sql/hive/src/main/scala/org/apache/spark/sql/hive/MetastoreRelation.scala

@@ -59,7 +59,7 @@ private[hive] case class MetastoreRelation(
     Objects.hashCode(databaseName, tableName, output)
   }
 
-  override protected def otherCopyArgs: Seq[AnyRef] = catalogTable :: sparkSession :: Nil
+  override protected def otherCopyArgs: Seq[AnyRef] = catalogTable :: client :: sparkSession :: Nil
 
   private def toHiveColumn(c: StructField): FieldSchema = {
     new FieldSchema(c.name, c.dataType.catalogString, c.getComment.orNull)

sql/hive/src/test/scala/org/apache/spark/sql/hive/MetastoreRelationSuite.scala

+/*
+* Licensed to the Apache Software Foundation (ASF) under one or more
+* contributor license agreements.  See the NOTICE file distributed with
+* this work for additional information regarding copyright ownership.
+* The ASF licenses this file to You under the Apache License, Version 2.0
+* (the "License"); you may not use this file except in compliance with
+* the License.  You may obtain a copy of the License at
+*
+*    http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+package org.apache.spark.sql.hive
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.catalyst.TableIdentifier
+import org.apache.spark.sql.catalyst.catalog.{CatalogStorageFormat, CatalogTable, CatalogTableType}
+import org.apache.spark.sql.types.{IntegerType, StructField, StructType}
+
+class MetastoreRelationSuite extends SparkFunSuite {
+  test("makeCopy and toJSON should work") {
+    val table = CatalogTable(
+      identifier = TableIdentifier("test", Some("db")),
+      tableType = CatalogTableType.VIEW,
+      storage = CatalogStorageFormat.empty,
+      schema = StructType(StructField("a", IntegerType, true) :: Nil))
+    val relation = MetastoreRelation("db", "test")(table, null, null)
+
+    // No exception should be thrown
+    relation.makeCopy(Array("db", "test"))
+    // No exception should be thrown
+    relation.toJSON
+  }
+}