[SPARK-12941][SQL][MASTER] Spark-SQL JDBC Oracle dialect fails to map string datatypes to Oracle VARCHAR datatype mapping

A test suite added for the bug fix -SPARK 12941; for the mapping of the StringType to corresponding in Oracle

manual tests done
(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: thomastechs <[email protected]>
Author: THOMAS SEBASTIAN <[email protected]>

Closes #11489 from thomastechs/thomastechs-12941-master-new.

(cherry picked from commit f6ac7c3)
Signed-off-by: Yin Huai <[email protected]>

Conflicts:
	sql/core/src/test/scala/org/apache/spark/sql/jdbc/JDBCSuite.scala

Author: thomastechs

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

@@ -501,4 +501,10 @@ class JDBCSuite extends SparkFunSuite with BeforeAndAfter with SharedSQLContext
     assert(rows(0).getAs[java.sql.Timestamp](2)
       === java.sql.Timestamp.valueOf("2002-02-20 11:22:33.543543"))
   }
+
+  test("SPARK 12941: The data type mapping for StringType to Oracle") {
+    val oracleDialect = JdbcDialects.get("jdbc:oracle://127.0.0.1/db")
+    assert(oracleDialect.getJDBCType(StringType).
+      map(_.databaseTypeDefinition).get == "VARCHAR2(255)")
+  }
 }