Repeated headers are not correctly sent when using a client

[allenhumphreys]

async def test_minimal_repro():
    repeated_headers = httpx.Headers([("repeated", "1"), ("repeated", "2")])
    assert repeated_headers.get_list("repeated") == ["1", "2"]
    async with httpx.AsyncClient() as client:
        response: httpx.Response = await client.get("https://google.com", headers=repeated_headers)
        assert response.request.headers.get_list("repeated") == ["1", "2"]

When using AsyncClient and attempting to pass repeated headers using the mulit-dict, the repeated header values are not retained.

The above code will fail with AssertionError: assert ['2'] == ['1', '2'].

The problem seems tp stem from the implementation of Headers.update which will result in only the last value for a header key being applied.