Fixed validation when trying to set an object instead of a value.

Let's say model validation is defined like this:
```
validation: {
  "name": {
    required: true,
    minLength: 1
  }
}
```

If we assigned an object to the model (empty or non-empty), here is what happened:
```
model.set("name", {});
model.isValid("name"); // Returns false
model.validate(); // Returns undefined

model.set("name", {"attributeKey": "attributeValue"});
model.isValid("name"); // Returns false
model.validate(); // Returns undefined
```

I changed it so that in these cases an error message will be returned.