[SPARK-19611][SQL] Preserve metastore field order when merging inferred schema

The ```HiveMetastoreCatalog.mergeWithMetastoreSchema()``` method added in #16944 may
not preserve the same field order as the metastore schema in some cases, which can cause
queries to fail. This change ensures that the metastore field order is preserved.

A test for ensuring that metastore order is preserved was added to ```HiveSchemaInferenceSuite.```
The particular failure usecase from #16944 was tested manually as well.

Author: Budde

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

@@ -356,13 +356,10 @@ private[hive] object HiveMetastoreCatalog {
       .filterKeys(!inferredSchema.map(_.name.toLowerCase).contains(_))
       .values
       .filter(_.nullable)
-
     // Merge missing nullable fields to inferred schema and build a case-insensitive field map.
     val inferredFields = StructType(inferredSchema ++ missingNullables)
       .map(f => f.name.toLowerCase -> f).toMap
-    StructType(metastoreFields.map { case(name, field) =>
-      field.copy(name = inferredFields(name).name)
-    }.toSeq)
+    StructType(metastoreSchema.map(f => f.copy(name = inferredFields(f.name).name)))
   } catch {
     case NonFatal(_) =>
       val msg = s"""Detected conflicting schemas when merging the schema obtained from the Hive

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

@@ -293,6 +293,27 @@ class HiveSchemaInferenceSuite
           StructField("firstField", StringType, nullable = true),
           StructField("secondField", StringType, nullable = true))))
     }.getMessage.contains("Detected conflicting schemas"))
+
+    // Schema merge should maintain metastore order.
+    assertResult(
+      StructType(Seq(
+        StructField("first_field", StringType, nullable = true),
+        StructField("second_field", StringType, nullable = true),
+        StructField("third_field", StringType, nullable = true),
+        StructField("fourth_field", StringType, nullable = true),
+        StructField("fifth_field", StringType, nullable = true)))) {
+      HiveMetastoreCatalog.mergeWithMetastoreSchema(
+        StructType(Seq(
+          StructField("first_field", StringType, nullable = true),
+          StructField("second_field", StringType, nullable = true),
+          StructField("third_field", StringType, nullable = true),
+          StructField("fourth_field", StringType, nullable = true),
+          StructField("fifth_field", StringType, nullable = true))),
+        StructType(Seq(
+          StructField("fifth_field", StringType, nullable = true),
+          StructField("third_field", StringType, nullable = true),
+          StructField("second_field", StringType, nullable = true))))
+    }
   }
 }