Don't poll for disconnects in BaseHTTPMiddleware via StreamingResponse (#2620)

Don't poll for disconnects in BaseHTTPMiddleware via StreamingResponse (#2620)

* Don't poll for disconnects in BaseHTTPMiddleware via StreamingResponse

Fixes #2516

* add test

* fmt

* Update tests/middleware/test_base.py

Co-authored-by: Mikkel Duif <mikkel@duifs.dk>
* add test for line now missing coverage

* more coverage, fix test

* add comment

* fmt

* tweak test

* fix

* fix coverage

* relint

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Co-authored-by: Mikkel Duif <mikkel@duifs.dk>