notFound handler raises an error on return
If I add a return to the notFound handler in a basic Nim server it throws an exception.

A minimal example code is
``` Nim
import happyx

serve("127.0.0.1", 5000):
  get "/":
    echo req
    return ""

  notFound:
    echo "Not found!"
    return ""
```

It throws

```
Not found!
An attempt was made to complete a Future more than once. Details:
  Future ID: 20
  Created in proc: handleRequest
  Stack trace to moment of creation:
    @[(procname: "simple2", line: 3, filename: "/home/charly/tmp/nim/rest/MetrotelAPI/simple2.nim"), (procname: "main", line: 372, filename: "/home/charly/.nimble/pkgs2/happyx-3.9.0-5476ae16b4bca92b898aca0551ecf5bf91e89cbb/happyx/ssr/server.nim"), (procname: "waitFor", line: 2022, filename: "/home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asyncdispatch.nim"), (procname: "poll", line: 1711, filename: "/home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asyncdispatch.nim"), (procname: "runOnce", line: 1452, filename: "/home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asyncdispatch.nim"), (procname: "processPendingCallbacks", line: 269, filename: "/home/charly/.choosenim/toolchains/nim-#devel/lib/system/threadimpl.nim"), (procname: "processRequest (Async)", line: 336, filename: "/home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asynchttpserver.nim"), (procname: "newFuture", line: 116, filename: "/home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asyncfutures.nim")]
  Stack trace to moment of secondary completion:
    Traceback (most recent call last)
    /home/charly/tmp/nim/rest/MetrotelAPI/simple2.nim(3) simple2
    /home/charly/.nimble/pkgs2/happyx-3.9.0-5476ae16b4bca92b898aca0551ecf5bf91e89cbb/happyx/ssr/server.nim(372) main
    /home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asyncdispatch.nim(2022) waitFor
    /home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asyncdispatch.nim(1711) poll
    /home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asyncdispatch.nim(1452) runOnce
    /home/charly/.choosenim/toolchains/nim-#devel/lib/system/threadimpl.nim(269) processPendingCallbacks
    /home/charly/tmp/nim/rest/MetrotelAPI/simple2.nim(10) handleRequest (Async)
    /home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asyncfutures.nim(211) complete
    /home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asyncfutures.nim(198) completeImpl
    /home/charly/.choosenim/toolchains/nim-#devel/lib/pure/asyncfutures.nim(160) checkFinished
```

