fix(websocket): do not swallow cancellations (#284)

Websocket client requests do not handle cancellation properly; they wrap
a `CancelledError` in an `RpcPostError`.
Any code that uses websocket requests can therefore no longer be
cancelled properly.

This change fixes the issue.

Author: markspanbroek

json_rpc/clients/websocketclient.nim

@@ -84,6 +84,8 @@ method request(
   client.withPendingFut(fut, id):
     try:
       await transport.send(reqData, Opcode.Binary)
+    except CancelledError as exc:
+      raise exc
     except CatchableError as exc:
       raise (ref RpcPostError)(msg: exc.msg, parent: exc)