Ensure Javadoc is generated using UTF-8 encoding

Fixes gh-23253

build.gradle

@@ -217,6 +217,7 @@ configure(subprojects - project(":spring-build-src")) { subproject ->
 	javadoc {
 		description = "Generates project-level javadoc for use in -javadoc jar"
 
+		options.encoding = "UTF-8"
 		options.memberLevel = org.gradle.external.javadoc.JavadocMemberLevel.PROTECTED
 		options.author = true
 		options.header = project.name

gradle/docs.gradle

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 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.
@@ -24,6 +24,7 @@ task api(type: Javadoc) {
 			it.tasks.getByName("jar")
 		}
 	}
+	options.encoding = "UTF-8"
 	options.memberLevel = org.gradle.external.javadoc.JavadocMemberLevel.PROTECTED
 	options.author = true
 	options.header = rootProject.description