[SPARK-4051] [SQL] [PySpark] Convert Row into dictionary

[davies]

Added a method to Row to turn row into dict:

>>> row = Row(a=1)
>>> row.asDict()
{'a': 1}

[SparkQA]

QA tests have started for PR 2896 at commit 8d97366.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2896 at commit 8d97366.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • raise TypeError("Cannot convert a Row class into dict")

[AmplabJenkins]

Test PASSed.
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https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/22044/
Test PASSed.

[liancheng]

LGTM, thanks :)

[JoshRosen]

This looks good to me, too. I guess it doesn't make sense for Row to extend dict, since it already extends tuple and uses tuple's indexing, etc, so this seems like a good approach. I'm going to merge this.