Promote SendRequest::pending to an OpaqueStreamRef. (#281)

Because `self.pending` doesn't necessarily get cleaned up in a timely fashion -
rather, only when the user calls `poll_ready()` - it was possible for it to
refer to a stream that has already been closed. This would lead to a panic the
next time that `poll_ready()` was called.

Instead, use an `OpaqueStreamRef`, bumping the refcount.

A change to an existing test is included which demonstrates the issue.

src/proto/mod.rs

@@ -9,7 +9,7 @@ mod streams;
 pub(crate) use self::connection::{Config, Connection};
 pub(crate) use self::error::Error;
 pub(crate) use self::peer::{Peer, Dyn as DynPeer};
-pub(crate) use self::streams::{Key as StreamKey, StreamRef, OpaqueStreamRef, Streams};
+pub(crate) use self::streams::{StreamRef, OpaqueStreamRef, Streams};
 pub(crate) use self::streams::{PollReset, Prioritized, Open};
 
 use codec::Codec;