Only use display_as property from response when defined

Hey,

I am using best_in_place as follows:
```ruby
best_in_place @issue, :summary
best_in_place @issue, :description, :display_as => :description_markdown
```

That causes a problem when the response includes a property called `display_as`. Since I am using the same path to update my property my response always includes a `display_as` property. I have added a new condition to the successHandler to only update the element with the `display_as` property when the data attribute `original-content` is set.

Please note that all the JS specs are failing after following the instructions from the README.:
```
ReferenceError: $ is not defined
```

I may need some advices on how to run the js specs. I am using Firefox 18.0.2

Thanks,
Tobias