title: How to get split values instead of bucket index in pyspark.bucketizer
body: <p>I'm trying to get the splits values as a result when using bucketizer in pyspark. 
  Currently the result contains the bucket's index:</p>
  
  <pre><code>data = [(0, -1.0), (1, 0.0), (2, 0.5), (3, 1.0), (4, 10.0),(5, 25.0),(6, 100.0),(7, 300.0),(8,float("nan"))]
  df = spark.createDataFrame(data, ["id", "value"])
  splits = [-float("inf"),0,0.001, 1, 5,10, 20, 30, 40, 50, 60, 70, 80, 90, 100, float("inf")]
  result_bucketizer = Bucketizer(splits=splits, inputCol="value",outputCol="result").setHandleInvalid("keep").transform(df)
  result_bucketizer.show()
  </code></pre>
  
  <p>The result is:</p>
  
  <pre><code>+---+-----+------+
  | id|value|result|
  +---+-----+------+
  |  0| -1.0|   0.0|
  |  1|  0.0|   1.0|
  |  2|  0.5|   2.0|
  |  3|  1.0|   3.0|
  |  4| 10.0|   5.0|
  |  5| 25.0|   6.0|
  |  6|100.0|  14.0|
  |  7|300.0|  14.0|
  |  8|  NaN|  15.0|
  +---+-----+------+
  </code></pre>
  
  <p>I want the result to be:</p>
  
  <pre><code>+---+-----+------+
  | id|value|result|
  +---+-----+------+
  |  0| -1.0|  -inf|
  |  1|  0.0|   0.0|
  |  2|  0.5| 0.001|
  |  3|  1.0|   1.0|
  |  4| 10.0|  10.0|
  |  5| 25.0|  20.0|
  |  6|100.0| 100.0|
  |  7|300.0| 100.0|
  |  8|  NaN|   NaN|
  +---+-----+------+ 
  </code></pre>