Fix Css autoupdate for pages with ROOT_URL_PATH_PREFIX set

Hi, I fixed an issue which broke the CSS autoupdate for me for a few months now. I thought it would be a different problem, some dynamic javascript required to properly init the template or somesuch, but today I decided to check for the source of the problem.

It seems like I found it, it is similar to this one: https://github.com/meteor/meteor/commit/0f4f77f8a20f34a035cfd2bce57041bfe170877f which got patched the same way.

The fix works for me, i tried it for projects without a ROOT_URL set as well, please check and, if possible, include it into the core! :)

Best wishes

Daniel