Make sure to correctly clone Buffer properties

The Buffer properties in the file object are not correctly handled, as a result, if a previous plugin (like metalsmith-include for instance) added a Buffer property, this one is not usable in the template of the *virtual* files (you get `[Object object]` instead of the expected string). This is because, lodash.clone does not take care of the Buffer properties but fortunately it's easy to fix with a lodash.clone callback as explained in https://github.com/lodash/lodash/issues/311.

I also added a related unit test.