mux-test: mux close (IO)
Removed `onException`, since the socket is already closed using
a bracket.
In `CloseOnRead` case ignore `Shutdown {}` - as in other cases. This
should fix:
```
mux close (IO): FAIL (0.08s)
*** Failed! Falsified (after 8 tests):
CloseOnRead
[]
<function>
-10
Right (Shutdown (Just (IOException withIODataPtr (recvBuf): invalid argument (An existing connection was forcibly closed by the remote host.) "recv errored")) (Failed (IOException withIODataPtr (recvBuf): invalid argument (An existing connection was forcibly closed by the remote host.) "recv errored")))
```