Support ASGI middleware chains by only yielding http.disconnect when response has been fully received

[tonybaloney]

Fixes a bug where http.disconnect would be sent even though the ASGI middleware hadn't received the whole response, which yielded empty responses.

This was shown with 2 or more pieces of FastAPI middleware because it would send them in chunks.

As per the ASGI spec, http.disconnect should be sent when:

Sent to the application if receive is called after a response has been sent or after the HTTP connection has been closed. This is mainly useful for long-polling, where you may want to trigger cleanup code if the connection closes early.

So the receive method has been changed to only send then when all response body has been added to the buffer.

Fixes https://github.com/Azure/azure-functions-python-worker/issues/1104

[codecov[bot]]

Codecov Report

Merging #153 (1f0926e) into dev (bd11d48) will decrease coverage by 0.05%. The diff coverage is 71.42%.

@@            Coverage Diff             @@
##              dev     #153      +/-   ##
==========================================
- Coverage   90.13%   90.08%   -0.06%     
==========================================
  Files          50       50              
  Lines        3002     3005       +3     
  Branches      400      401       +1     
==========================================
+ Hits         2706     2707       +1     
- Misses        220      221       +1     
- Partials       76       77       +1     

Flag	Coverage Δ
unittests	90.04% <71.42%> (-0.06%)	:arrow_down:

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Impacted Files	Coverage Δ
azure/functions/_http_asgi.py	90.42% <71.42%> (-1.89%)	:arrow_down:

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