SyncClient: update session with transport's headers

[shtrom]

This works around graphql-python/gql#613.

fixes: #202

[Eijebong]

This got 2 issues.

It's not in the right place. It should be in _get_gql_session or doing GQL -> REST -> GQL with the same object ends up with the second gql call behaving different.

And secondly, this overrides all headers instead of updating them. So we lose default headers (notably Accept-Encoding and User-Agent).

Something like this test shows both well:

def test_headers(responses, sync_client):
    defaults = set(requests.Session().headers)
    print("Default:", defaults)
    responses.post(GITHUB_GRAPHQL_ENDPOINT, status=200, json={"data": {"foo": "bar"}})
    responses.get(f"{GITHUB_API_ENDPOINT}/octocat", status=200, json={"answer": 42})

    sync_client.execute("query { foo }")
    before = dict(responses.calls[-1].request.headers)
    print("Before:", before)

    sync_client.get("/octocat")
    sync_client.execute("query { foo }")
    after = dict(responses.calls[-1].request.headers)
    print("After:", after)

    session = sync_client._get_requests_session()
    assert defaults <= set(session.headers)
    assert before == after
    assert session.headers["Accept"] == "application/vnd.github+json"
    assert session.headers["Authorization"] == f"Bearer {sync_client.auth._token}"

You get:

Default: {'Accept-Encoding', 'User-Agent', 'Accept', 'Connection'}
Before: {'User-Agent': 'python-requests/2.34.2', 'Accept-Encoding': 'gzip, deflate, br, zstd', 'Accept': 'application/vnd.github+json', 'Connection': 'keep-alive', 'Authorization': 'Bearer abc', 'Content-Length': '24', 'Content-Type': 'application/json'}
After: {'Accept': 'application/vnd.github+json', 'Authorization': 'Bearer abc', 'Content-Length': '24', 'Content-Type': 'application/json'}

I'm 95% sure the UA missing gets saved by urllib3 later on and that it'll work anyway (github requires a UA) but it's a very weird behavior and I don't want to have to debug something 6 months from now because the UA changes depending on request order