Use === when checking argument existence

In the callWatchers function, the 

```javascript
if (prop) {
```

must be replaced with

```
if (prop !== undefined) {
```

to check the existence of the prop parameter, otherwise, if prop is the index 0 of an array, the "if (prop)" will be false and all the watchers of the array (i.e. those of other properties) will be executed, until finding one property with no watchers.