Ignore BrokenResourceError in BaseHTTPMiddleware

ASGI specifies that send is a no-op when the connection is closed.

HTTPMiddleware uses anyio streams which will raise
an exception when the connection is closed.

So we need to ignore the exception in our send function.

Fixes encode#1284

starlette/middleware/base.py

@@ -1,4 +1,5 @@
 import typing
+from contextlib import suppress
 
 import anyio
 
@@ -25,9 +26,13 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         async def call_next(request: Request) -> Response:
             send_stream, recv_stream = anyio.create_memory_object_stream()
 
+            async def docile_stream_send(data):
+                with suppress(anyio.BrokenResourceError):
+                    await send_stream.send(data)
+
             async def coro() -> None:
                 async with send_stream:
-                    await self.app(scope, request.receive, send_stream.send)
+                    await self.app(scope, request.receive, docile_stream_send)
 
             task_group.start_soon(coro)