Fix integration test for Python 3

[laike9m]

Originally this two tests test_adapter_received_values and test_adapter_sending_values would fail in Python 3 due to connection header not being removed, which is a bug of the h2_safe_headers function.

headers(class HTTPHeaderMap) stores bytes because we call to_bytestring_tuple when initiating it. While in Python 2 bytes is equivalent to str, and h2_safe_headers can find 'connection' successfully, in Python 3 b'connection' != 'connection', so this header is not removed.

Some affected tests are also modified.

[Lukasa]

This looks good to me, thanks so much! ✨

[laike9m]

BTW how to join the Hyper organization, created certain amount of PRs?

[Lukasa]

@laike9m Strictly, this repository is not a part of the Hyper organisation yet. I appreciate that that's confusing, but for various boring organisational reasons I haven't brought it in yet. However, literally any PR against any of the projects in this org would qualify you.