[SPARK-5775][SQL] BugFix: GenericRow cannot be cast to SpecificMutableRow when nested data and partitioned table

[yhuai]

This one is only used to triggered jenkins to test my changes. I will make a pull request to @liancheng's branch and #4792 is the PR for the fix.

[SparkQA]

Test build #28028 has started for PR 4798 at commit f412cd0.

• This patch merges cleanly.

[SparkQA]

Test build #28028 has finished for PR 4798 at commit f412cd0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/28028/
Test PASSed.