Allow protocol relative URLs in CssLink Transformer

This commit allows the use of "protcol relative URLs" (i.e. URLs without
scheme, starting with `//`), often used to serve resources automatically
from https or http with third party domains.

This syntax is allowed by RFC 3986.

Issue: SPR-12632

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/CssLinkResourceTransformer.java

@@ -123,7 +123,8 @@ public class CssLinkResourceTransformer extends ResourceTransformerSupport {
 
 	private boolean hasScheme(String link) {
 		int schemeIndex = link.indexOf(":");
-		return schemeIndex > 0 && !link.substring(0, schemeIndex).contains("/");
+		return (schemeIndex > 0 && !link.substring(0, schemeIndex).contains("/"))
+				|| link.indexOf("//") == 0;
 	}
 
 

spring-webmvc/src/test/java/org/springframework/web/servlet/resource/CssLinkResourceTransformerTests.java

@@ -99,7 +99,8 @@ public class CssLinkResourceTransformerTests {
 		TransformedResource transformedResource = (TransformedResource) resource;
 
 		String expected = "@import url(\"http://example.org/fonts/css\");\n" +
-				"body { background: url(\"file:///home/spring/image.png\") }";
+				"body { background: url(\"file:///home/spring/image.png\") }\n" +
+				"figure { background: url(\"//example.org/style.css\")}";
 		String result = new String(transformedResource.getByteArray(), "UTF-8");
 		result = StringUtils.deleteAny(result, "\r");
 		assertEquals(expected, result);
@@ -108,6 +109,8 @@ public class CssLinkResourceTransformerTests {
 				.resolveUrlPath("http://example.org/fonts/css", Arrays.asList(externalCss));
 		Mockito.verify(resolverChain, Mockito.never())
 				.resolveUrlPath("file:///home/spring/image.png", Arrays.asList(externalCss));
+		Mockito.verify(resolverChain, Mockito.never())
+				.resolveUrlPath("//example.org/style.css", Arrays.asList(externalCss));
 	}
 
 	@Test

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

@@ -1,2 +1,3 @@
 @import url("http://example.org/fonts/css");
-body { background: url("file:///home/spring/image.png") }
+body { background: url("file:///home/spring/image.png") }
+figure { background: url("//example.org/style.css")}