Fix StackSender.Send to always honor the `W` and `R` modes, by:
1) returning a valid Sender and Receiver when the respective flags are set, and
2) returning a value which doesn't need to be Closed, when they are not.
This also moves `NopSender` and `NopReceiver` out of `utils`, to prevent
a circular dependency.
Signed-off-by: Solomon Hykes <solomon@docker.com>
Solomon Hykes