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/streams/mod.rs

@@ -12,7 +12,6 @@ mod streams;
 pub(crate) use self::prioritize::Prioritized;
 pub(crate) use self::recv::Open;
 pub(crate) use self::send::PollReset;
-pub(crate) use self::store::Key;
 pub(crate) use self::streams::{StreamRef, OpaqueStreamRef, Streams};
 
 use self::buffer::Buffer;