Filenames with @ cause SASS issues

[eburton]

Don't know if this is even still maintained or updated, but filenames that include the @ symbol cause issues when trying to include the map in your SASS project. I'm assuming this is due to how SASS uses the @ symbol.

A simple fix is to simply wrap quotes around the 'css_class_name' value which forms the key for each set of image data in the generated file.