handle BrokenStreamError during shutdown grace period

Fixes #70

Author: belm0

examples/trio-server.py

@@ -168,6 +168,8 @@ async def shutdown_and_clean_up(self):
                     got = await self.stream.receive_some(MAX_RECV)
                     if not got:
                         break
+            except trio.BrokenStreamError:
+                pass
             finally:
                 await self.stream.aclose()