HADOOP-15854. AuthToken Use StringBuilder instead of StringBuffer.

Contributed by Beluga Behr.

hadoop-common-project/hadoop-auth/src/main/java/org/apache/hadoop/security/authentication/util/AuthToken.java

@@ -128,7 +128,7 @@ public class AuthToken implements Principal {
    * Generates the token.
    */
   private void generateToken() {
-    StringBuffer sb = new StringBuffer();
+    StringBuilder sb = new StringBuilder();
     sb.append(USER_NAME).append("=").append(getUserName()).append(ATTR_SEPARATOR);
     sb.append(PRINCIPAL).append("=").append(getName()).append(ATTR_SEPARATOR);
     sb.append(TYPE).append("=").append(getType()).append(ATTR_SEPARATOR);