Fix bug in `css` method

When a callback function is provided as the second argument to the `css`
method, it should be invoked with each element's index and the value of
the specified CSS property (not the element itself).

From the jQuery API documentation on `css`:

> # .css( propertyName, function )
>
> **propertyName**
> Type: String
> A CSS property name.
>
> **function**
> Type: Function( Integer index, String value ) => String or Number
> A function returning the value to set. `this` is the current element.
> Receives the index position of the element in the set and the old
> value as arguments.

http://api.jquery.com/css/#css-propertyName-function
Merge pull request #583 from jugglinmike/fix-css

Fix bug in `css` method