Merge branch '5.1.x'

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -874,7 +874,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	/**
 	 * Set the {@link Locale} of the content language,
 	 * as specified by the {@literal Content-Language} header.
-	 * <p>Use {@code set(CONTENT_LANGUAGE, ...)} if you need
+	 * <p>Use {@code set(CONTENT_LANGUAGE, list)} if you need
 	 * to set multiple content languages.</p>
 	 * @since 5.0
 	 */

spring-web/src/main/java/org/springframework/http/codec/EncoderHttpMessageWriter.java

@@ -119,7 +119,8 @@ public class EncoderHttpMessageWriter<T> implements HttpMessageWriter<T> {
 
 		if (inputStream instanceof Mono) {
 			HttpHeaders headers = message.getHeaders();
-			return Mono.from(body)
+			return body
+					.singleOrEmpty()
 					.switchIfEmpty(Mono.defer(() -> {
 						headers.setContentLength(0);
 						return message.setComplete().then(Mono.empty());