[SPARK-19726][SQL] Faild to insert null timestamp value to mysql using spark jdbc

## What changes were proposed in this pull request?

when creating table like following:
> create table timestamp_test(id int(11), time_stamp timestamp not null default current_timestamp);

The result of Excuting "insert into timestamp_test values (111, null)" is different between Spark and JDBC.
```
mysql> select * from timestamp_test;
+------+---------------------+
| id   | time_stamp          |
+------+---------------------+
|  111 | 1970-01-01 00:00:00 | -> spark
|  111 | 2017-06-27 19:32:38 | -> mysql
+------+---------------------+
2 rows in set (0.00 sec)
```
   Because in such case ```StructField.nullable``` is false, so the generated codes of ```InvokeLike``` and ```BoundReference``` don't check whether the field is null or not. Instead, they directly use ```CodegenContext.INPUT_ROW.getLong(1)```, however, ```UnsafeRow.setNullAt(1)``` will put 0 in the underlying memory.

   The PR will ```always``` set ```StructField.nullable```  true after obtaining metadata from jdbc connection, Since we can insert null to not null timestamp column in MySQL. In this way, spark will propagate null to underlying DB engine, and let DB to choose how to process NULL.

## How was this patch tested?

Added tests.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: YIHAODIAN\wangshuangshuang <wangshuangshuang@yihaodian.com>
Author: Shuangshuang Wang <wsszone@gmail.com>

Closes #18445 from shuangshuangwang/SPARK-19726.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JDBCRDD.scala

@@ -59,7 +59,7 @@ object JDBCRDD extends Logging {
       try {
         val rs = statement.executeQuery()
         try {
-          JdbcUtils.getSchema(rs, dialect)
+          JdbcUtils.getSchema(rs, dialect, alwaysNullable = true)
         } finally {
           rs.close()
         }

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JdbcUtils.scala

@@ -266,10 +266,14 @@ object JdbcUtils extends Logging {
   /**
    * Takes a [[ResultSet]] and returns its Catalyst schema.
    *
+   * @param alwaysNullable If true, all the columns are nullable.
    * @return A [[StructType]] giving the Catalyst schema.
    * @throws SQLException if the schema contains an unsupported type.
    */
-  def getSchema(resultSet: ResultSet, dialect: JdbcDialect): StructType = {
+  def getSchema(
+      resultSet: ResultSet,
+      dialect: JdbcDialect,
+      alwaysNullable: Boolean = false): StructType = {
     val rsmd = resultSet.getMetaData
     val ncols = rsmd.getColumnCount
     val fields = new Array[StructField](ncols)
@@ -290,7 +294,11 @@ object JdbcUtils extends Logging {
             rsmd.getClass.getName == "org.apache.hive.jdbc.HiveResultSetMetaData" => true
         }
       }
-      val nullable = rsmd.isNullable(i + 1) != ResultSetMetaData.columnNoNulls
+      val nullable = if (alwaysNullable) {
+        true
+      } else {
+        rsmd.isNullable(i + 1) != ResultSetMetaData.columnNoNulls
+      }
       val metadata = new MetadataBuilder()
         .putString("name", columnName)
         .putLong("scale", fieldScale)

sql/core/src/test/scala/org/apache/spark/sql/jdbc/JDBCWriteSuite.scala

@@ -24,6 +24,7 @@ import scala.collection.JavaConverters.propertiesAsScalaMapConverter
 
 import org.scalatest.BeforeAndAfter
 
+import org.apache.spark.SparkException
 import org.apache.spark.sql.{AnalysisException, DataFrame, Row, SaveMode}
 import org.apache.spark.sql.catalyst.parser.ParseException
 import org.apache.spark.sql.execution.datasources.jdbc.{JDBCOptions, JdbcUtils}
@@ -506,4 +507,11 @@ class JDBCWriteSuite extends SharedSQLContext with BeforeAndAfter {
         "schema struct<name:string,id:int>"))
     }
   }
+
+  test("SPARK-19726: INSERT null to a NOT NULL column") {
+    val e = intercept[SparkException] {
+      sql("INSERT INTO PEOPLE1 values (null, null)")
+    }.getMessage
+    assert(e.contains("NULL not allowed for column \"NAME\""))
+  }
 }