Use StringBuilder.append(char) where possible

To slightly improve performance, this commit switches to
StringBuilder.append(char) instead of StringBuilder.append(String)
whenever we append a single character to a StringBuilder.

Closes gh-27098

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/handler/HtmlFileTransportHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -75,7 +75,7 @@ public class HtmlFileTransportHandler extends AbstractHttpSendingTransportHandle
 				);
 
 		while (sb.length() < MINIMUM_PARTIAL_HTML_CONTENT_LENGTH) {
-			sb.append(" ");
+			sb.append(' ');
 		}
 		PARTIAL_HTML_CONTENT = sb.toString();
 	}