[issue15851] Lib/robotparser.py doesn't accept setting a user agent string, instead it uses the default.

[Xiang Zhang]

Changes by Xiang Zhang <angwerzx at 126.com>:


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versions: +Python 3.7 -Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue15851>
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