Leverage RouterFunctions.Builder in RouterFunctionDsl

Closes gh-22423

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/RouterFunctionDslTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -120,7 +120,7 @@ class RouterFunctionDslTests {
 				.verifyComplete()
 	}
 
-	@Test
+	@Test(expected = IllegalStateException::class)
 	fun emptyRouter() {
 		router { }
 	}