Sleep::downcast_mut_pin() should not extend lifetime

[burakemir]

Could you provide safety docs that explain why it is ok to extend the lifetime to static here?

hyper/src/rt/timer.rs

Line 107 in 90eb95f

pub fn downcast_mut_pin<T>(self: Pin<&mut Self>) -> Option<Pin<&'static mut T>>

[sfackler]

That extension should definitely not be there.