Use enum for MessageCodesResolver format

Fixes gh-670

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration.java

@@ -31,6 +31,7 @@ import org.springframework.beans.factory.ListableBeanFactory;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.boot.autoconfigure.AutoConfigureAfter;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnExpression;
@@ -137,7 +138,7 @@ public class WebMvcAutoConfiguration {
 		private Integer cachePeriod;
 
 		@Value("${spring.mvc.message-codes-resolver.format:}")
-		private String messageCodesResolverFormat = "";
+		private DefaultMessageCodesResolver.Format messageCodesResolverFormat = null;
 
 		@Value("${spring.mvc.locale:}")
 		private String locale = "";
@@ -204,10 +205,10 @@ public class WebMvcAutoConfiguration {
 		@ConditionalOnExpression("'${spring.mvc.message-codes-resolver.format:}' != ''")
 		public MessageCodesResolver messageCodesResolver() {
 			DefaultMessageCodesResolver resolver = new DefaultMessageCodesResolver();
-			resolver.setMessageCodeFormatter(DefaultMessageCodesResolver.Format.valueOf(messageCodesResolverFormat));
+			resolver.setMessageCodeFormatter(this.messageCodesResolverFormat);
 			return resolver;
 		}
-		
+
 		@Override
 		public void addFormatters(FormatterRegistry registry) {
 			for (Converter<?, ?> converter : getBeansOfType(Converter.class)) {