Fix prepending of loader.browserRootURL to root-relative url

Previously background-image: url(/foo.png) would be left as-is
despite loader.browerRootURL being configured, causing browsers
to fail to load it when the stylesheet is being loaded as an
object url.

Author: papandreou

css-plugin-base-builder.js

@@ -122,15 +122,19 @@ exports.bundle = function(loads, compileOpts, outputOpts) {
     }
   }), atUrl({
     url: function(fileName, decl, from, dirname, to, options, result) {
-      if (absUrl(fileName) || fileName.match(/^%23/))
+      if ((absUrl(fileName) && fileName.charAt(0) !== '/') || fileName.match(/^%23/))
         return fileName;
 
       // dirname may be renormalized to cwd
       if (dirname.substr(0, cwd.length + 1) === cwd + path.sep)
         dirname = path.resolve(baseURLPath, dirname.substr(cwd.length + 1));
 
       if (loader.rootURL)
-        return (loader.browserRootURL || '/') + path.relative(loader.rootURL, path.join(dirname, fileName)).replace(/\\/g, '/');
+        if (fileName.charAt(0) === '/') {
+          return (loader.browserRootURL || '/') + fileName.replace(/\\/g, '/').replace(/\//, '');
+        } else {
+          return (loader.browserRootURL || '/') + path.relative(loader.rootURL, path.join(dirname, fileName)).replace(/\\/g, '/');
+        }
       else
         return path.relative(baseURLPath, path.join(dirname, fileName)).replace(/\\/g, '/');
     }