Fix URL rebase (fixes #39)

[mgreter]

This change would fix the issues I'm having in #39.
Not sure if this is sane or changing usefull old behavior!

Edit: refactored the fix a little bit more.

Btw. npm test does not work on windows:

> gulp-concat-css@2.3.0 test D:\github-forks\gulp-concat-css
> node_modules/mocha/bin/mocha test/*.js --reporter spec

'node_modules' is not recognized as an internal or external command,
operable program or batch file.
npm ERR! Test failed.  See above for more details.

Had to use node node_modules\mocha\bin\mocha --reporter spec

[antur84]

Thanks, this also solved a very similar issue we were facing, it did not introduce any known new issues.