dataset.unpersist() doesn't update storage memory stats

Steps to reproduce:

1) Start a Spark cluster, and check the storage memory value from the Spark Web UI "Executors" tab (it should be equal to zero if you just started)

2) Run:

val df = spark.sqlContext.range(1, 1000000000)
df.cache()
df.count()
df.unpersist(true)

3) Check the storage memory value again, now it's equal to 1GB

Looks like the memory is actually released, but stats aren't updated. This issue makes cluster management more complicated.