Handle setKeyValue case

[cherifGsoul]

This fixes the case of setting properties with canReflect.setKeyValue that dispatches events more than once, the issue was caused after the changes to observable class fields:

class MyType extends ObservableObject {
}

const myType = new MyType();

canReflect.onKeyValue(myType, 'foo', (newVal) => {
    console.log(newVal); // -> 4 will be logged once
});

canReflect.setKeyValue(myType, 'foo', 4);

Fixes #35

[phillipskevin]

This could be MyType.instances.has(myType). It's fine either way.

[phillipskevin]

Node 12 requirement is curious, but go ahead publish.