Compass is not deleting old sprites after assigning generated_images_dir

[mattscilipoti]

If you assign a value to generated_images_dir, the old sprite files are not deleted when new sprites are created.

Example:
Given "app/assets/images/common" has many images
And config.compass.generated_images_dir = 'public/assets'
And Compass has already generated "/public/assets/common-sff832da50b.png"

When we add a new image to "app/assets/images/common"
And we "@import "common/*.png";
Then Compass should generate a new "common" sprite file
And "public/assets/common-sff832da50b.png" should not exist

Observation: the new file is generated, but the old Sprite is not deleted.
Note: this does work correctly if config.generated_images_dir is the default value.

Tested on 0.12.0 and ref:e2b8409646c35e2cb47de3c0ab9d68bac5e9ddca.