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/recv.rs

@@ -59,7 +59,6 @@ struct Indices {
 
 impl<B, P> Recv<B, P>
 where
-    B: Buf,
     P: Peer,
 {
     pub fn new(config: &Config) -> Self {
@@ -468,6 +467,7 @@ where
     ) -> Poll<(), io::Error>
     where
         T: AsyncWrite,
+        B: Buf,
     {
         if let Some(stream_id) = self.refused {
             try_ready!(dst.poll_ready());
@@ -493,6 +493,7 @@ where
     ) -> Poll<(), io::Error>
     where
         T: AsyncWrite,
+        B: Buf,
     {
         // Send any pending connection level window updates
         try_ready!(self.send_connection_window_update(dst));
@@ -510,6 +511,7 @@ where
     ) -> Poll<(), io::Error>
     where
         T: AsyncWrite,
+        B: Buf,
     {
         if let Some(incr) = self.flow.unclaimed_capacity() {
             let frame = frame::WindowUpdate::new(StreamId::zero(), incr);
@@ -541,6 +543,7 @@ where
     ) -> Poll<(), io::Error>
     where
         T: AsyncWrite,
+        B: Buf,
     {
         loop {
             // Ensure the codec has capacity