Nesting CassetteContextDecorators does not work properly

[colonelpanic8]

Nesting any use of CassetteContextDecorators will result in complete unpatching after the first context is exited. This is because the reset function in patch.py indiscriminately sets all the patches back to the original state that was found in the module at import time.

Patching is a pretty tricky problem and I think that doing it by hand amounts to reinventing the wheel in some sense. I think that the best way to fix this issue is to rely on the mock library to do all of the patching.

Here are two tests that illustrate the problem:

@mock.patch('vcr.cassette.requests_match', _mock_requests_match)
@mock.patch('vcr.cassette.Cassette.can_play_response_for', return_value=True)
@mock.patch('vcr.cassette.Cassette._save', return_value=True)
def test_nesting_cassette_context_managers(*args):
    first_response = {'body': {'string': b'first_response'}, 'headers': {},
                      'status': {'message': 'm', 'code': 200}}

    second_response = copy.deepcopy(first_response)
    second_response['body']['string'] = b'second_response'

    with Cassette.use('test') as first_cassette, \
        mock.patch.object(first_cassette, 'play_response', return_value=first_response):
        assert_get_response_body_is('first_response')

        # Make sure a second cassette can supercede the first
        with Cassette.use('test') as second_cassette, \
        mock.patch.object(second_cassette, 'play_response', return_value=second_response):
            assert_get_response_body_is('second_response')

        # Now the first cassette should be back in effect
        assert_get_response_body_is('first_response')


def test_nesting_context_managers_by_checking_references_of_http_connection():
    with Cassette.use('test'):
        first_cassette_HTTPConnection = httplib.HTTPConnection
        with Cassette.use('test'):
            pass
        assert httplib.HTTPConnection is first_cassette_HTTPConnection

and here is the output that they produce:
https://gist.github.com/anonymous/f51bdd0334ee5fdb33bc

[colonelpanic8]

is it a problem that I use the multiple context manage with block syntax here? That wasn't available until python 2.7.

I think we should maybe add contextlib2 as a dependency so that we can have a good way to enter multiple context manager.

[colonelpanic8]

This was fixed by #108.