Fix #317 handling of preserved comments in CSS

[timhunt]

I hope this is OK. It is my first pull request here.

Your CONTRIBUTING.md was very clear (thanks!) but I may still have missed something. If so, sorry, please let me know, and I will be happy to fix.

[timhunt]

Thanks to scrutinizer for poniting out that this applies to JS too. Amended fix coming up.

[timhunt]

Sorry, I don't understand whay appveyor is trying to do, and I can't work out what the logs mean. If I need to fix something, please explain.

I felt it was clearer to leave the change as two separate commits, but if you disagree, let me know what you would like instead.

This fix may explain a number of the open issues about chunks of files not getting minimised.

[davidscotson]

Issues #288, #283, #294 and #310 seem like they might be fixed by this PR.

[timhunt]

I was just wondering if we could get this fix landed? If you are waiting for me to do something, then please help me to understand what needs to be done. Thank you.