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/stream.rs

@@ -220,8 +220,8 @@ where
             // There are no more outstanding references to the stream
             self.ref_count == 0 &&
             // The stream is not in any queue
-            !self.is_pending_send && !self.is_pending_send_capacity
-            && !self.is_pending_accept && !self.is_pending_window_update
+            !self.is_pending_send && !self.is_pending_send_capacity &&
+            !self.is_pending_accept && !self.is_pending_window_update
     }
 
     pub fn assign_capacity(&mut self, capacity: WindowSize) {