告警邮件固定使用 UTF-8 编码格式

xxl-job-admin/src/main/java/com/xxl/job/admin/core/util/MailUtil.java

 			}
 
 			email.setAuthenticator(new DefaultAuthenticator(XxlJobAdminConfig.getAdminConfig().getMailUsername(), XxlJobAdminConfig.getAdminConfig().getMailPassword()));
-			email.setCharset(Charset.defaultCharset().name());
+			email.setCharset("UTF-8");
 
 			email.setFrom(XxlJobAdminConfig.getAdminConfig().getMailUsername(), XxlJobAdminConfig.getAdminConfig().getMailSendNick());
 			email.addTo(toAddress);