SPARK-1677: allow user to disable output dir existence checking

https://issues.apache.org/jira/browse/SPARK-1677

For compatibility with older versions of Spark it would be nice to have an option `spark.hadoop.validateOutputSpecs` (default true)  for the user to disable the output directory existence checking

Author: CodingCat <zhunansjtu@gmail.com>

Closes #947 from CodingCat/SPARK-1677 and squashes the following commits:

7930f83 [CodingCat] miao
c0c0e03 [CodingCat] bug fix and doc update
5318562 [CodingCat] bug fix
13219b5 [CodingCat] allow user to disable output dir existence checking
[SPARK-2036] [SQL] CaseConversionExpression should check if the evaluated value is null.

`CaseConversionExpression` should check if the evaluated value is `null`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #982 from ueshin/issues/SPARK-2036 and squashes the following commits:

61e1c54 [Takuya UESHIN] Add check if the evaluated value is null.