Wrong URL about base64 encoded data

Do not prepend assets path to base64 encoded data in css image url.

e.g
When using Yii-Debug, some image url using base64 encoded data will always be prepended assets path too, please ignore them.
refer to css: [@vendor/yiisoft/yii2-debug/assets/toolbar.css](https://github.com/yiisoft/yii2-debug/blob/master/assets/toolbar.css), line 15 and line 173

HTML 404 error
```
https://www.example.com/assets/1b70d06/data:image/svg+xml;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAAoCAYAAAA/tpB3AAAAAXNSR0IArs4c6QAAAAZiS0dEAP8A/wD/oL2nkwAAAAlwSFlzAAALEwAACxMBAJqcGAAAAAd0SU1FB94BBQwuINct3v0AAAAjSURBVAjXY3j79u1/JgYGBgYkgpGRkYEYMSpKUGLK+/fvGQAaDAb6F86IsAAAAABJRU5ErkJggg==
```