Adds test for Supplier functions via config in server-webmvc

See gh-3646

spring-cloud-gateway-server-mvc/src/test/java/org/springframework/cloud/gateway/server/mvc/config/FunctionHandlerConfigTests.java

@@ -18,6 +18,7 @@ package org.springframework.cloud.gateway.server.mvc.config;
 
 import java.util.Locale;
 import java.util.function.Function;
+import java.util.function.Supplier;
 
 import org.junit.jupiter.api.Test;
 
@@ -63,6 +64,18 @@ public class FunctionHandlerConfigTests {
 			.isEqualTo("HELLO");
 	}
 
+	@Test
+	public void testSupplierFunctionWorks() {
+		restClient.get()
+			.uri("/supplierfunction")
+			.accept(MediaType.TEXT_PLAIN)
+			.exchange()
+			.expectStatus()
+			.isOk()
+			.expectBody(String.class)
+			.isEqualTo("hello");
+	}
+
 	@SpringBootConfiguration
 	@EnableAutoConfiguration
 	protected static class TestConfiguration {
@@ -72,6 +85,11 @@ public class FunctionHandlerConfigTests {
 			return s -> s.toUpperCase(Locale.ROOT);
 		}
 
+		@Bean
+		Supplier<String> hello() {
+			return () -> "hello";
+		}
+
 	}
 
 }

spring-cloud-gateway-server-mvc/src/test/resources/application-functionhandlerconfigtests.yml

@@ -10,7 +10,11 @@ spring.cloud.gateway.mvc:
       predicates:
         - Path=/templatedfunction/{fnName}
         - Method=POST
-
+    testsupplierfunction:
+      uri: fn:hello
+      predicates:
+        - Path=/supplierfunction
+        - Method=GET
 logging:
   level:
     org.springframework.cloud.gateway.server.mvc: TRACE