Do not allow free override of computed properties

At this point when one do `obj.computed = true` it persistently shadows getter with _direct_ value.
It can lead to hard to track bugs if done by mistake.

Such operation should be allowed only via dedicated method e.g.:

``` javascript
obj.overrideComputed('computed', true);
```

Related issue: https://github.com/medikoo/dbjs-persistence/issues/22
