[issue26045] Improve error message for http.client when posting unicode string

[Terry J. Reedy]

Changes by Terry J. Reedy <tjreedy at udel.edu>:


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versions:  -Python 3.2, Python 3.3, Python 3.4, Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26045>
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