import_asset produces incorrect destination_path for asset

I'm using `sprockets.import_asset` in an `after_configuration` block  which references a css file loaded in through a gem, like so:

```ruby
after_configuration do
  sprockets.import_asset "gem.css" # source file is gem.css.scss
end
```

In my scenario, the source file is `gem.css.scss`.The file name of the assets that is generated ends up being `gem.css.scss`, which doesn't seem right.

Other assets in site source that the same extensions are converted correctly with out the `.scss`.

I have a failing test over on my fork if you want to pull it and check it out: https://github.com/jordanandree/middleman-sprockets/tree/import-asset-ext