[SPARK-5464] Fix help() for Python DataFrame instances

[JoshRosen]

This fixes an exception that prevented users from calling help() on Python DataFrame instances.

[JoshRosen]

I don't think we need this check because getattr can only be called with strings:

>>> foo = []
>>> getattr(foo, 123)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: getattr(): attribute name must be string

[SparkQA]

Test build #26341 has started for PR 4278 at commit 08f95f7.

• This patch merges cleanly.

[JoshRosen]

Row already had this check in its __getattr__ call, so I think it's safe to do the same thing here, too.

[rxin]

cc @davies

[JoshRosen]

This is the case that threw an exception. The other two cases below are just to improve our test-coverage, since those classes also have custom __getattr__ methods.

[davies]

This looks good to me, thanks for fix them.

[SparkQA]

Test build #26341 has finished for PR 4278 at commit 08f95f7.

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

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/26341/
Test PASSed.

[rxin]

Thanks. merging this in master!