[SPARK-4150][PySpark] return self in rdd.setName

[mengxr]

Then we can do rdd.setName('abc').cache().count().

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• This patch merges cleanly.

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Test build #22515 has finished for PR 3011 at commit 4ac3bbd.

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• This patch adds no public classes.

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[mengxr]

test this please

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Test build #22525 has started for PR 3011 at commit 10d0d60.

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Test build #22525 has finished for PR 3011 at commit 10d0d60.

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[davies]

good catch, LGTM!

[JoshRosen]

LGTM, so I'm merging this. Thanks!