Fix: option 'defaultValue' for 'date' property should set 'DEFAULT = now() 

consider a model
```js
Project.hasMany('members',
        User,
        { since: {  type: 'date',
                    time: true,
                    defaultValue: Date.now
                 }},
        { reverse: 'projects'} );

```

I gets an error 'column "since" is of type timestamp without time zone but default expression is of type bigint' while syncing the databse with my postgresSql db
setting
```js
defaultValue =  function(){ return new Date(); }
```
didn't solved the problem. So I fixed it. 
I am not sure whether it is the right method