Impl Sink for With should buffer the items

[hosyvietanh]

I recently faced an issue when using Sink to send websocket messages.
I used sink.with in order to modify the msg before actually sending it. The items are not buffered which caused some messages being lost.
https://github.com/rust-lang-nursery/futures-rs/blob/e9cd539cc977fda701f6f4679ee435fa66986ed8/futures-util/src/sink/with.rs#L171

I'm not sure if I understand the purpose of with correctly, but I think it should buffer the items when calling start_send instead of replacing the old ones.

[Nemo157]

There is definitely a bug because With::poll is not calling self.sink.poll_ready before it attempts to send the buffered item, but if the underlying sink isn’t ready to accept it then that should result in an error rather than missing items, so there may be something else I’m missing.

[Nemo157]

Actually, I guess With itself doesn’t guarantee an error if you start sending before it’s ready, so maybe the underlying sink could be similar. Unfortunately I don’t see any way for With to create an error if start_send is called while it has a buffered item...

[cramertj]

I think #1880 is all we're planning to do on this-- let me know if more is wanted here.