Fixed number validation error. Fixes #1359

Changed core.js line 1153
To
```javascript
return this.optional(element) ||
/^(?:-?\d+|-?\d{1,3}(?:,\d{3})+)?(?:\.\d+)?$/.test(value);
```
From
```javascript
return this.optional(element) ||
/^-?(?:\d+|\d{1,3}(?:,\d{3})+)?(?:\.\d+)?$)?$/.test(value);
```