[SPARK-7180] [SPARK-8090] [SPARK-8091] Fix a number of SerializationDebugger bugs and limitations

This PR solves three SerializationDebugger issues.
* SPARK-7180 - SerializationDebugger fails with ArrayOutOfBoundsException
* SPARK-8090 - SerializationDebugger does not handle classes with writeReplace correctly
* SPARK-8091 - SerializationDebugger does not handle classes with writeObject method

The solutions for each are explained as follows
* SPARK-7180 - The wrong slot desc was used for getting the value of the fields in the object being tested.
* SPARK-8090 - Test the type of the replaced object.
* SPARK-8091 - Use a dummy ObjectOutputStream to collect all the objects written by the writeObject() method, and then test those objects as usual.

I also added more tests in the testsuite to increase code coverage. For example, added tests for cases where there are not serializability issues.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #6625 from tdas/SPARK-7180 and squashes the following commits:

c7cb046 [Tathagata Das] Addressed comments on docs
ae212c8 [Tathagata Das] Improved docs
304c97b [Tathagata Das] Fixed build error
26b5179 [Tathagata Das] more tests.....92% line coverage
7e2fdcf [Tathagata Das] Added more tests
d1967fb [Tathagata Das] Added comments.
da75d34 [Tathagata Das] Removed unnecessary lines.
50a608d [Tathagata Das] Fixed bugs and added support for writeObject