Proposed fix for issue #2360

I don't entirely understand how the tests are setup, so there are some
failing tests still but they appear to be isolated to base64 encoding
options.

lib/less/tree/url.js

@@ -23,7 +23,7 @@ URL.prototype.eval = function (context) {
     if (!this.isEvald) {
         // Add the base path if the URL is relative
         rootpath = this.currentFileInfo && this.currentFileInfo.rootpath;
-        if (rootpath && typeof val.value === "string" && context.isPathRelative(val.value)) {
+        if (rootpath && typeof val.value === "string" && !/^'?data:/.test(val.value) && context.isPathRelative(val.value)) {
             if (!val.quote) {
                 rootpath = rootpath.replace(/[\(\)'"\s]/g, function(match) { return "\\"+match; });
             }