[FIX] store: unobserve detached objects

closes #27

src/store.ts

@@ -137,6 +137,7 @@ export function makeObserver(): Observer {
           );
         }
         if (newVal !== value) {
+          unobserve(value);
           value = newVal;
           observe(newVal, obj);
           obj.__owl__.rev!++;
@@ -229,6 +230,12 @@ export function makeObserver(): Observer {
     }
   }
 
+  function unobserve(target: any) {
+    if (target !== null && typeof target === "object") {
+      delete target.__owl__;
+    }
+  }
+
   return observer;
 }