[SPARK-2860][SQL] Fix coercion of CASE WHEN.

Author: Michael Armbrust <michael@databricks.com>

Closes #1785 from marmbrus/caseNull and squashes the following commits:

126006d [Michael Armbrust] better error message
2fe357f [Michael Armbrust] Fix coercion of CASE WHEN.
[SPARK-2859] Update url of Kryo project in related docs

JIRA Issue: https://issues.apache.org/jira/browse/SPARK-2859

Kryo project has been migrated from googlecode to github, hence we need to update its URL in related docs such as tuning.md.

Author: Guancheng (G.C.) Chen <chenguancheng@gmail.com>

Closes #1782 from gchen/kryo-docs and squashes the following commits:

b62543c [Guancheng (G.C.) Chen] update url of Kryo project