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/store.rs

@@ -103,10 +103,6 @@ where
         }
     }
 
-    pub fn contains_id(&self, id: &StreamId) -> bool {
-        self.ids.contains_key(id)
-    }
-
     pub fn find_mut(&mut self, id: &StreamId) -> Option<Ptr<B, P>> {
         let key = match self.ids.get(id) {
             Some(key) => *key,