[SPARK-16750][FOLLOW-UP][ML] Add transformSchema for StringIndexer/VectorAssembler and fix failed tests.

## What changes were proposed in this pull request?
This is follow-up for #14378. When we add ```transformSchema``` for all estimators and transformers, I found there are tests failed for ```StringIndexer``` and ```VectorAssembler```. So I moved these parts of work separately in this PR, to make it more clear to review.
The corresponding tests should throw ```IllegalArgumentException``` at schema validation period after we add ```transformSchema```. It's efficient that to throw exception at the start of ```fit``` or ```transform``` rather than during the process.

## How was this patch tested?
Modified unit tests.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #14455 from yanboliang/transformSchema.

2 files changed, 12 insertions, 4 deletions

diff --git a/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala b/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala
index c221d4aa55..b478fea5e7 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala
@@ -120,12 +120,20 @@ class StringIndexerSuite
 
   test("StringIndexerModel can't overwrite output column") {
     val df = spark.createDataFrame(Seq((1, 2), (3, 4))).toDF("input", "output")
+    intercept[IllegalArgumentException] {
+      new StringIndexer()
+        .setInputCol("input")
+        .setOutputCol("output")
+        .fit(df)
+    }
+
     val indexer = new StringIndexer()
       .setInputCol("input")
-      .setOutputCol("output")
+      .setOutputCol("indexedInput")
       .fit(df)
+
     intercept[IllegalArgumentException] {
-      indexer.transform(df)
+      indexer.setOutputCol("output").transform(df)
     }
   }
 
diff --git a/mllib/src/test/scala/org/apache/spark/ml/feature/VectorAssemblerSuite.scala b/mllib/src/test/scala/org/apache/spark/ml/feature/VectorAssemblerSuite.scala
index 14973e79bf..561493fbaf 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/feature/VectorAssemblerSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/feature/VectorAssemblerSuite.scala
@@ -74,10 +74,10 @@ class VectorAssemblerSuite
     val assembler = new VectorAssembler()
       .setInputCols(Array("a", "b", "c"))
       .setOutputCol("features")
-    val thrown = intercept[SparkException] {
+    val thrown = intercept[IllegalArgumentException] {
       assembler.transform(df)
     }
-    assert(thrown.getMessage contains "VectorAssembler does not support the StringType type")
+    assert(thrown.getMessage contains "Data type StringType is not supported")
   }
 
   test("ML attributes") {