[SPARK-7318] [STREAMING] DStream cleans objects that are not closures

I added a check in `ClosureCleaner#clean` to fail fast if this is detected in the future. tdas

Author: Andrew Or <andrew@databricks.com>

Closes #5860 from andrewor14/streaming-closure-cleaner and squashes the following commits:

8e971d7 [Andrew Or] Do not throw exception if object to clean is not closure
5ee4e25 [Andrew Or] Fix tests
eed3390 [Andrew Or] Merge branch 'master' of github.com:apache/spark into streaming-closure-cleaner
67eeff4 [Andrew Or] Add tests
a4fa768 [Andrew Or] Clean the closure, not the RDD