[SPARK-18210][ML] Pipeline.copy does not create an instance with the same UID

## What changes were proposed in this pull request?

Motivation:
`org.apache.spark.ml.Pipeline.copy(extra: ParamMap)` does not create an instance with the same UID. It does not conform to the method specification from its base class `org.apache.spark.ml.param.Params.copy(extra: ParamMap)`

Solution:
- fix for Pipeline UID
- introduced new tests for `org.apache.spark.ml.Pipeline.copy`
- minor improvements in test for `org.apache.spark.ml.PipelineModel.copy`

## How was this patch tested?

Introduced new unit test: `org.apache.spark.ml.PipelineSuite."Pipeline.copy"`
Improved existing unit test: `org.apache.spark.ml.PipelineSuite."PipelineModel.copy"`

Author: Wojciech Szymanski <wk.szymanski@gmail.com>

Closes #15759 from wojtek-szymanski/SPARK-18210.

mllib/src/main/scala/org/apache/spark/ml/Pipeline.scala

@@ -169,7 +169,7 @@ class Pipeline @Since("1.4.0") (
   override def copy(extra: ParamMap): Pipeline = {
     val map = extractParamMap(extra)
     val newStages = map(stages).map(_.copy(extra))
-    new Pipeline().setStages(newStages)
+    new Pipeline(uid).setStages(newStages)
   }
 
   @Since("1.2.0")

mllib/src/test/scala/org/apache/spark/ml/PipelineSuite.scala

@@ -101,13 +101,31 @@ class PipelineSuite extends SparkFunSuite with MLlibTestSparkContext with Defaul
     }
   }
 
+  test("Pipeline.copy") {
+    val hashingTF = new HashingTF()
+      .setNumFeatures(100)
+    val pipeline = new Pipeline("pipeline").setStages(Array[Transformer](hashingTF))
+    val copied = pipeline.copy(ParamMap(hashingTF.numFeatures -> 10))
+
+    assert(copied.uid === pipeline.uid,
+      "copy should create an instance with the same UID")
+    assert(copied.getStages(0).asInstanceOf[HashingTF].getNumFeatures === 10,
+      "copy should handle extra stage params")
+  }
+
   test("PipelineModel.copy") {
     val hashingTF = new HashingTF()
       .setNumFeatures(100)
-    val model = new PipelineModel("pipeline", Array[Transformer](hashingTF))
+    val model = new PipelineModel("pipelineModel", Array[Transformer](hashingTF))
+      .setParent(new Pipeline())
     val copied = model.copy(ParamMap(hashingTF.numFeatures -> 10))
-    require(copied.stages(0).asInstanceOf[HashingTF].getNumFeatures === 10,
+
+    assert(copied.uid === model.uid,
+      "copy should create an instance with the same UID")
+    assert(copied.stages(0).asInstanceOf[HashingTF].getNumFeatures === 10,
       "copy should handle extra stage params")
+    assert(copied.parent === model.parent,
+      "copy should create an instance with the same parent")
   }
 
   test("pipeline model constructors") {