pathsend causing issues with BaseHTTPMiddleware

[aersam]

Hi guys

This is a follow up of https://github.com/emmett-framework/granian/issues/320

I'm not entirely sure if this is really a bug of starlette, or more of a feature that's missing or so.

On my staging environment, I did receive an assertion error:

 File "/tmp/8dc89326e92b51b/antenv/lib/python3.12/site-packages/starlette/middleware/base.py", line 173, in body_stream
2024-06-10T09:58:53.052818649Z     assert message["type"] == "http.response.body"

As I could not reproduce it locally, I monkey-patched starlette and printed an error message with details:

2024-06-10T09:58:53.052814649Z   File "/tmp/8dc89326e92b51b/antenv/lib/python3.12/site-packages/starlette/middleware/base.py", line 173, in body_stream
2024-06-10T09:58:53.052818649Z     assert message["type"] == "http.response.body", f"Unexpected message of type {message['type']} from class {type(self)}: {message}"  # noqa: E501
2024-06-10T09:58:53.052821649Z     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2024-06-10T09:58:53.052824449Z AssertionError: Unexpected message of type http.response.pathsend from class <class 'starlette.middleware.base.BaseHTTPMiddleware'>: {'type': 'http.response.pathsend', 'path': 'frontend_app/output/index.html'}

I think this happens because my server does support the pathsend extension, however middlware that act's on it and just derives from BaseHTTPMiddleware will fail, since it cannot handle this.

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