Misleading read_ready impl for VecDeque and &[u8]

[skilvingr]

Hi, I'm currently using a VecDeque to test a crate built around this crate.

At the moment the method read_ready returns always Ok(true), even if the queue is empty.
This results in possible infinite loops if a code uses a while loop with read_ready as exit condition.

I think that the returned value should depend upon the occupied space for both VecDeque and &[u8] implementations.