[HOTFIX] [SQL] Fixes DataFrameWriter.mode(String)

We forgot an assignment there.

/cc rxin

Author: Cheng Lian <lian@databricks.com>

Closes #6212 from liancheng/fix-df-writer and squashes the following commits:

711fbb0 [Cheng Lian] Adds a test case
3b72d78 [Cheng Lian] Fixes DataFrameWriter.mode(String)

sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala

@@ -55,7 +55,7 @@ final class DataFrameWriter private[sql](df: DataFrame) {
    * @since 1.4.0
    */
   def mode(saveMode: String): DataFrameWriter = {
-    saveMode.toLowerCase match {
+    this.mode = saveMode.toLowerCase match {
       case "overwrite" => SaveMode.Overwrite
       case "append" => SaveMode.Append
       case "ignore" => SaveMode.Ignore

sql/core/src/test/scala/org/apache/spark/sql/sources/SaveLoadSuite.scala

@@ -75,6 +75,13 @@ class SaveLoadSuite extends DataSourceTest with BeforeAndAfterAll {
     checkLoad()
   }
 
+  test("save with string mode and path, and load") {
+    conf.setConf(SQLConf.DEFAULT_DATA_SOURCE_NAME, "org.apache.spark.sql.json")
+    path.createNewFile()
+    df.write.mode("overwrite").save(path.toString)
+    checkLoad()
+  }
+
   test("save with path and datasource, and load") {
     conf.setConf(SQLConf.DEFAULT_DATA_SOURCE_NAME, "not a source name")
     df.write.json(path.toString)