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 2014-08-01 at 2 08 24 pm](https://cloud.githubusercontent.com/assets/3682072/3785558/3201076e-19c6-11e4-97c8-409937a8fc84.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)