Verify that the FakeOpener doesn't use a different codepath than standard HTTP responses

[mineo]

After looking at #208 I'm wondering if using StringIO in the FakeOpener doesn't change the behaviour a bit because resp in

python-musicbrainzngs/musicbrainzngs/musicbrainz.py

Line 673 in c2e7391

resp = _safe_read(opener, req, body)

is then of type str instead of bytes. This is hidden by parse_message which calls bytes_to_elementtree right at the beginning, which calls _unicode, which is is basically a noop for strs. For the standard HTTPResponse.read(), which returns bytes, the codepath is the same up to _unicode, which in that case is not a noop.