fix: resolve issue with computed property method called twice (#13)
fix: resolve issue with computed property method called twice (#13)

1) Computed property method `Model[callback](ctx.instance)` called twice.
To avoid this behaviour I have used `Promise.resolve`. It normalizes Promises and non-Promises.
See http://stackoverflow.com/questions/27746304/how-do-i-tell-if-an-object-is-a-promise

2) It also fixes the issue when computed property returns with `then` (e.g. `{then:true}`) but not actually the Promise.
fullcube/loopback-ds-computed-mixin