Create close coroutine only when scheduling it.

Otherwise, this can lead to:

    RuntimeWarning: coroutine 'Connection.close' was never awaited

Fix #1647.

Author: aaugustin

docs/faq/client.rst

@@ -103,7 +103,7 @@ You can close the connection.
 Here's an example that terminates cleanly when it receives SIGTERM on Unix:
 
 .. literalinclude:: ../../example/faq/shutdown_client.py
-    :emphasize-lines: 10-12
+    :emphasize-lines: 10-14
 
 How do I disable TLS/SSL certificate verification?
 --------------------------------------------------

example/faq/shutdown_client.py

@@ -9,7 +9,9 @@ async def client():
     async with connect("ws://localhost:8765") as websocket:
         # Close the connection when receiving SIGTERM.
         loop = asyncio.get_running_loop()
-        loop.add_signal_handler(signal.SIGTERM, loop.create_task, websocket.close())
+        def close():
+            return loop.create_task(websocket.close())
+        loop.add_signal_handler(signal.SIGTERM, close)
 
         # Process messages received on the connection.
         async for message in websocket: