StreamRef sends RST_STREAM on drop (#109)

This PR modifies the `Drop` implementation for `StreamRef` to reset the underlying stream if it is the last reference to that stream. Since both `Stream` and `Body` are internally just a `StreamRef`, this means they will both reset the stream on drop; thus, this closes #100.

The assertion that the store no longer contains the dropped stream ID at the end of the `Drop` method  had to be removed, as the stream has to be reset from inside of a `transition` block (which now manages releasing that ID for us), and the `transition` closure moves the value of `stream`, making the assertion no longer possible.

Modifications to some of the tests in `flow_control.rs` were also necessary, in order to prevent `StreamRef`s from being dropped too early.

src/proto/streams/prioritize.rs

@@ -46,7 +46,6 @@ pub(crate) struct Prioritized<B> {
 
 impl<B, P> Prioritize<B, P>
 where
-    B: Buf,
     P: Peer,
 {
     pub fn new(config: &Config) -> Prioritize<B, P> {
@@ -101,7 +100,10 @@ where
         frame: frame::Data<B>,
         stream: &mut store::Ptr<B, P>,
         task: &mut Option<Task>,
-    ) -> Result<(), UserError> {
+    ) -> Result<(), UserError>
+    where
+        B: Buf,
+    {
         let sz = frame.payload().remaining();
 
         if sz > MAX_WINDOW_SIZE as usize {
@@ -369,6 +371,7 @@ where
     ) -> Poll<(), io::Error>
     where
         T: AsyncWrite,
+        B: Buf,
     {
         // Ensure codec is ready
         try_ready!(dst.poll_ready());
@@ -422,7 +425,10 @@ where
         &mut self,
         store: &mut Store<B, P>,
         dst: &mut Codec<T, Prioritized<B>>,
-    ) -> bool {
+    ) -> bool
+    where
+        B: Buf,
+    {
         trace!("try reclaim frame");
 
         // First check if there are any data chunks to take back
@@ -485,7 +491,10 @@ where
         store: &mut Store<B, P>,
         max_len: usize,
         counts: &mut Counts<P>,
-    ) -> Option<Frame<Prioritized<B>>> {
+    ) -> Option<Frame<Prioritized<B>>>
+    where
+        B: Buf,
+    {
         trace!("pop_frame");
 
         loop {