Changed "extend" loop by adding hasOwnProperty()

Changed the ternary operator to a simple `if()` w/ `Object.prototype.hasOwnProperty()` because

* the condition `opts` was always `true` and thus unnecessary
* `opts[property]` also will do a scope chain lookup and can cause trouble when using `true` or `false` as a value.

Some infos:
[http://andrew.hedges.name/experiments/in/](http://andrew.hedges.name/experiments/in/)
[Mozilla Developer Network](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty)
[underscore.js annotated source code | Section 85](http://underscorejs.org/docs/underscore.html#section-85)