Fixing panic if the Future wakes up after returning Poll::Ready

[Pauan]

Some things like futures::channel::mpsc::UnboundedSender will trigger wakeups after returning Poll::Ready, so this fixes it so it ignores the spurious wakeups, rather than panicking.

The diff looks scary, but it's really just a tiny bit of reorganization (changing future.as_mut().unwrap_throw() into if let Some(future) = borrow.as_mut() { ... })

It looks a lot less scary if you disable whitespace in the diffs.

[alexcrichton]

Good catch!