Ignore external resources in CssLinkResourceTransormer

Prior to this commit, the CssLinkResourceTransformer would transform
"external resources", i.e. resources not served by the web application.

This commit only allows transformation for resources which path don't
contain scheme such as "file://" or "http://". Only relative and
absolute paths for resources served by the webapp are valid.

Issue: SPR-11860

spring-webmvc/src/test/resources/org/springframework/web/servlet/resource/test/external.css

@@ -0,0 +1,2 @@
+@import url("http://example.org/fonts/css");
+body { background: url("file:///home/spring/image.png") }

spring-webmvc/src/test/resources/org/springframework/web/servlet/resource/test/main.css

@@ -7,5 +7,3 @@
 @import 'foo.css';
 
 body { background: url("images/image.png") }
-
-li { list-style: url(http://www.example.com/redball.png) disc }