[SPARK-16118][MLLIB] add getDropLast to OneHotEncoder

## What changes were proposed in this pull request?

We forgot the getter of `dropLast` in `OneHotEncoder`

## How was this patch tested?

unit test

Author: Xiangrui Meng <meng@databricks.com>

Closes #13821 from mengxr/SPARK-16118.

mllib/src/main/scala/org/apache/spark/ml/feature/OneHotEncoder.scala

@@ -59,6 +59,10 @@ class OneHotEncoder @Since("1.4.0") (@Since("1.4.0") override val uid: String) e
     new BooleanParam(this, "dropLast", "whether to drop the last category")
   setDefault(dropLast -> true)
 
+  /** @group getParam */
+  @Since("2.0.0")
+  def getDropLast: Boolean = $(dropLast)
+
   /** @group setParam */
   @Since("1.4.0")
   def setDropLast(value: Boolean): this.type = set(dropLast, value)

mllib/src/test/scala/org/apache/spark/ml/feature/OneHotEncoderSuite.scala

@@ -49,7 +49,9 @@ class OneHotEncoderSuite
     val encoder = new OneHotEncoder()
       .setInputCol("labelIndex")
       .setOutputCol("labelVec")
-      .setDropLast(false)
+    assert(encoder.getDropLast === true)
+    encoder.setDropLast(false)
+    assert(encoder.getDropLast === false)
     val encoded = encoder.transform(transformed)
 
     val output = encoded.select("id", "labelVec").rdd.map { r =>