In tests, I tweaked model.parse(opt) to be model.parse(attrs) for descriptiveness

I was looking through the tests to get a better understanding of backbone.  I figured the tests would be a great source of documentation for the granular aspects.  :)

For the model parse() tests, I saw that the input arg was `obj`.  I thought `attrs` would be a bit more descriptive.  This makes it align with the source.  What do you guys think?  

In the end, this is a very microscopic change, but this is my first commit to open source, so I figured I'd try to see what happens!  :)