Fixes #1739 - _.extend check for own properties

I believe _.extend is intended to remain primitive in nature (as per issue #123), so this may be an unnecessary change.

Checks for the source object's own properties before extending a property. It includes the code from Dimik's comment resolving issue #1739. One contributed test; All tests pass.

Screenshots highlight lines of code added to underscore.js and object tests:
<b>underscore.js:</b>
![screen shot 2014-07-31 at 7 53 36 pm](https://cloud.githubusercontent.com/assets/3682072/3774373/8eb4f684-1929-11e4-83a5-6675a886cc81.png)
<b>test/objects.js:</b>
![screen shot 2014-07-31 at 7 54 28 pm](https://cloud.githubusercontent.com/assets/3682072/3774378/9ba9e610-1929-11e4-94e9-6f8565113aa7.png)
<b>Passing tests:</b>
![screen shot 2014-07-31 at 8 16 02 pm](https://cloud.githubusercontent.com/assets/3682072/3774399/3bcd7e40-192a-11e4-8025-1fc1e5fced0b.png)