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.

tests/prioritization.rs

@@ -202,7 +202,12 @@ fn send_data_receive_window_update() {
             let payload = vec![0; frame::DEFAULT_INITIAL_WINDOW_SIZE as usize];
             stream.send_data(payload.into(), true).unwrap();
 
-            h2.unwrap()
+            h2.map(|h2| {
+                // keep `stream` from being dropped in order to prevent
+                // it from sending an RST_STREAM frame.
+                std::mem::forget(stream);
+                h2
+            }).unwrap()
         });
 
     let mock = mock.assert_client_handshake().unwrap()