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, line 15 and line 173
Merge pull request #4 from ychongsaytc/patch-1

Wrong URL about base64 encoded data