Python 3 JSON dump encoding test

If relying on the response content auto-serialization of dict to JSON, Python 3 evaluations of the response content fail with a type error due to how requests.utils.guess_json_utf() attempts to figure out if the content is UTF-8 or not.

This commit adds a unit test that fails when using this functionality, causing TypeError exception:

    ======================================================================
    ERROR: test_get (tests.Python3EncodingTest)
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    Traceback (most recent call last):
      File "/Users/gmr/Dropbox/Source/httmock/tests.py", line 183, in test_get
        self.assertEqual(self.content, response.json())
      File "/usr/local/Cellar/python3/3.3.2/Frameworks/Python.framework/Versions/3.3/lib/python3.3/site-packages/requests/models.py", line 687, in json
        encoding = guess_json_utf(self.content)
      File "/usr/local/Cellar/python3/3.3.2/Frameworks/Python.framework/Versions/3.3/lib/python3.3/site-packages/requests/utils.py", line 518, in guess_json_utf
        nullcount = sample.count(_null)
    TypeError: Can't convert 'bytes' object to str implicitly