Send an AssetEvent when modifying using get_id_mut

Fixes bevyengine#321

crates/bevy_asset/src/assets.rs

@@ -65,7 +65,7 @@ impl<T: Resource> Assets<T> {
     }
 
     pub fn get_id_mut(&mut self, id: HandleId) -> Option<&mut T> {
-        self.assets.get_mut(&Handle::from_id(id))
+        self.get_mut(&Handle::from_id(id))
     }
 
     pub fn get(&self, handle: &Handle<T>) -> Option<&T> {