[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.

(cherry picked from commit 6e821e3d1ae1ed23459bc7f1098510b968130152)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[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
(cherry picked from commit ac3440f4f3c4b79070ffec7db0b08ad062b4df90)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>