Fix ReadBuffer tests to run and test the right things

Fix ReadBuffer tests to run and test the right things

Without 'Test' in the class name, the test did not run at all. It would
have caught #615.

StringIO.closed is not assignable under Python 3, but we can just
close() the fake stream to make it True.

Reading from response.raw gives back bytes, not unicode, so the fake
stream in the test case should do the same.

And finally, ReadBuffer itself yields unicode, not UTF-8-encoded bytes.

See https://github.com/tweepy/tweepy/pull/635 for the actual fix that
this tests.