Fix ExceptionWrapper::with_exception to support lvalue ref functors

Fix ExceptionWrapper::with_exception to support lvalue ref functors

Summary: [Folly] Fix `ExceptionWrapper::with_exception` to support lvalue ref functors.

Ex:

    auto handler = [&](const std::runtime_error& e) { handle_runtime_error(e); };
    exception_wrapper wrap = get_some_exception_wrapper_from_context();
    wrap.with_exception(handler); // broken before this diff, fixed after

Reviewed By: markisaa

Differential Revision: D2698680

fb-gh-sync-id: 4976ba08e3601e22891d00d79a5dae5118887b71