Keep default Thymeleaf media types in reactive support

This commit removes the default configuration value previously set
for `spring.thymeleaf.reactive.media-types` since this value overrides
the defaults provided by Thymeleaf.

This value does not drive the default media type used by views, but
rather all media types that the templating engine should support.

Fixes gh-9134

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/thymeleaf/ThymeleafAutoConfiguration.java

@@ -19,6 +19,7 @@ package org.springframework.boot.autoconfigure.thymeleaf;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.LinkedHashMap;
+import java.util.List;
 
 import javax.annotation.PostConstruct;
 
@@ -55,6 +56,7 @@ import org.springframework.context.ApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.Ordered;
+import org.springframework.http.MediaType;
 import org.springframework.util.MimeType;
 import org.springframework.web.servlet.resource.ResourceUrlEncodingFilter;
 
@@ -252,8 +254,10 @@ public class ThymeleafAutoConfiguration {
 			ThymeleafReactiveViewResolver resolver = new ThymeleafReactiveViewResolver();
 			resolver.setTemplateEngine(templateEngine);
 			resolver.setDefaultCharset(this.properties.getEncoding());
-			resolver.setSupportedMediaTypes(
-					this.properties.getReactive().getMediaTypes());
+			final List<MediaType> mediaTypes = this.properties.getReactive().getMediaTypes();
+			if (mediaTypes != null) {
+				resolver.setSupportedMediaTypes(mediaTypes);
+			}
 			resolver.setExcludedViewNames(this.properties.getExcludedViewNames());
 			resolver.setViewNames(this.properties.getViewNames());
 			if (this.properties.getReactive().getMaxChunkSize() > 0) {

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/thymeleaf/ThymeleafProperties.java

@@ -17,8 +17,6 @@
 package org.springframework.boot.autoconfigure.thymeleaf;
 
 import java.nio.charset.Charset;
-import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 
 import org.springframework.boot.context.properties.ConfigurationProperties;
@@ -227,8 +225,7 @@ public class ThymeleafProperties {
 		/**
 		 * Media types supported by the view technology.
 		 */
-		private List<MediaType> mediaTypes = new ArrayList<>(
-				Collections.singletonList(MediaType.TEXT_HTML));
+		private List<MediaType> mediaTypes;
 
 		public List<MediaType> getMediaTypes() {
 			return this.mediaTypes;