Replace StringBuffer usages with StringBuilder

Fixes gh-718

spring-session/src/main/java/org/springframework/session/web/http/CookieHttpSessionStrategy.java

@@ -251,18 +251,18 @@ public final class CookieHttpSessionStrategy
 			return sessionIds.values().iterator().next();
 		}
 
-		StringBuffer buffer = new StringBuffer();
+		StringBuilder sb = new StringBuilder();
 		for (Map.Entry<String, String> entry : sessionIds.entrySet()) {
 			String alias = entry.getKey();
 			String id = entry.getValue();
 
-			buffer.append(alias);
-			buffer.append(this.serializationDelimiter);
-			buffer.append(id);
-			buffer.append(this.serializationDelimiter);
+			sb.append(alias);
+			sb.append(this.serializationDelimiter);
+			sb.append(id);
+			sb.append(this.serializationDelimiter);
 		}
-		buffer.deleteCharAt(buffer.length() - 1);
-		return buffer.toString();
+		sb.deleteCharAt(sb.length() - 1);
+		return sb.toString();
 	}
 
 	public void onInvalidateSession(HttpServletRequest request,

spring-session/src/test/java/org/springframework/session/web/http/CookieHttpSessionStrategyTests.java

@@ -705,19 +705,19 @@ public class CookieHttpSessionStrategyTests {
 	}
 
 	private String createSessionCookieValue(long size) {
-		StringBuffer buffer = new StringBuffer();
+		StringBuilder sb = new StringBuilder();
 
 		for (long i = 0; i < size; i++) {
 			String hex = Long.toHexString(i);
-			buffer.append(hex);
-			buffer.append(" ");
-			buffer.append(i);
+			sb.append(hex);
+			sb.append(" ");
+			sb.append(i);
 			if (i < size - 1) {
-				buffer.append(" ");
+				sb.append(" ");
 			}
 		}
 
-		return buffer.toString();
+		return sb.toString();
 	}
 
 	public void setCookieName(String cookieName) {