message: Add test case for with_storage
diff: @@ -118,6 +118,14 @@ mod test {
  // it is updated to whatever we set it with
  instance.set_gas(123456);
  assert_eq!(123456, instance.get_gas());
  + }
  +
  + #[test]
  + #[should_panic]
  + fn with_context_safe_for_panic() {
  + // this should fail with the assertion, but not cause a double-free crash (issue #59)
  + let storage = MockStorage::new();
  + let instance = Instance::from_code(CONTRACT, storage).unwrap();
  instance.with_storage(|_store| assert_eq!(1, 2));
  }
  