Fix output extensions

Patch for #3

Gulp has changed the passed file object to be a [wrapped version](https://github.com/wearefractal/gulp-util/blob/master/lib/File.js). This changes the way relative paths are retrieved (a [js getter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Working_with_Objects?redirectlocale=en-US&redirectslug=Web%2FJavaScript%2FGuide%2FObsolete_Pages%2FCreating_New_Objects%2FDefining_Getters_and_Setters#Defining_getters_and_setters) rather than a property). The clone lib does not copy over getters https://github.com/pvorb/node-clone/issues/25

This fix removes the cloning of the passed file object and just returns the same one with modified contents

@dlmanning