fix(httpx): don't save response if filtered to None

[alekseik1]

VCR py fails to record response made by starlette (and FastAPI as well) TestClient #628.
It seems like the problem hides in this assert that is triggered by this func call.

I took a quick look and it looks like we're trying to record response to cassete object here.
But at the same time, here we see that whenever filters render response to None, a real request is always performed. So, why should we record real response if we don't use it anyway? I made a small check with should_save_response variable that runs same filters and skips recording if it will never be replayed.

There's one catch though. Image following scenario:

1. User sets ignore_hosts=["some.real.host.org"], runs tests and gets his cassettes.
2. Neither of cassettes contains requests to "some.real.host.org" since we skipped writing them (by should_save_response var).
3. Now, user decides to set ignore_hosts=[] and rerun his tests.
4. His cassettes are no longer valid because of a missing request to "some.real.host.org". Such behavior is introduced by this PR.

Does it look sensible?

[alekseik1]

damn, this anyio stuff is really hard to debug. By trials and errors, I found out that run_coroutine_threadsafe seem to fix problems with TestClient. My idea is that starlette's TestClient might lead to race conditions when reading response because TestClient uses threads internally (see here).

Unfortunately, any attempt to reproduce bug in test environment failed - all tests were green without my fix applied. I believe that problems with TestClient start on large codebase, when FastAPI has lots of Tasks/Futures running in handles.