add content type for compatibility issue

apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/spi/defaultimpl/DefaultSsoHeartbeatHandler.java

@@ -14,6 +14,7 @@ public class DefaultSsoHeartbeatHandler implements SsoHeartbeatHandler {
   @Override
   public void doHeartbeat(HttpServletRequest request, HttpServletResponse response) {
     try {
+      response.setContentType("text/plain;charset=utf-8");
       response.getWriter().write("default sso heartbeat handler");
     } catch (IOException e) {
     }