[SysApps] Fix bug when adding a method that returns a Promise

The BindingObject should create a Promise for every method call. The
current code was using the same instance for every call and it will
obviously fail after the first method invocation.

Also update the tests so it would have caught the bug in the first place.
Merge pull request #1312 from tmpsantos/promise_method_fix

[SysApps] Fix bug when adding a method that returns a Promise