receive_data() stores every DATA frame in both _body_chunks (list)
and request_body (BytesIO). The receive() closure in
_handle_http2_request() has two read paths: a streaming path that
pops from _body_chunks, and a fast path that reads from BytesIO.
After the streaming path consumed the body, the fast path could
re-read the same data from BytesIO because body_received was never
set in the streaming return path. This caused the application to
receive a doubled request body (e.g. 18 bytes sent, 36 bytes
received), breaking JSON parsing with "Extra data" errors.
Fix: set body_received = True in the streaming path when
_body_complete is True, preventing the fast path from re-reading
already-consumed data.
Fixes#3558
