Check if scheme is localhost and fail

Fixes gh-2919

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/route/Route.java

@@ -187,9 +187,11 @@ public B uri(URI uri) {
 			this.uri = uri;
 			String scheme = this.uri.getScheme();
 			Assert.hasText(scheme, "The parameter [" + this.uri + "] format is incorrect, scheme can not be empty");
-			if (!scheme.equalsIgnoreCase("forward")) {
-				Assert.hasText(this.uri.getHost(),
-						"The parameter [" + this.uri + "] format is incorrect, host can not be empty");
+			if (scheme.equalsIgnoreCase("localhost")) {
+				// common error
+				// TODO: find a general way to detect without breaking existing behavior
+				throw new IllegalArgumentException(
+						"The parameter [" + this.uri + "] format is incorrect, scheme can not be localhost");
 			}
 			if (this.uri.getPort() < 0 && scheme.startsWith("http")) {
 				// default known http ports