title: how to get user birthday from facebook sdk?
body: <p>How to get user birthday by <code>user.getBirthday()</code> ? </p>
  
  <pre><code>Session.openActiveSession(this, true, new Session.StatusCallback() {
        // callback when session changes state
        @Override
        public void call(Session session, SessionState state, Exception exception) {
          if (session.isOpened()) {
  
            // make request to the /me API
            Request.executeMeRequestAsync(session, new Request.GraphUserCallback() {
  
              // callback after Graph API response with user object
              @Override
              public void onCompleted(GraphUser user, Response response) {
                if (user != null) {
                  TextView welcome = (TextView) findViewById(R.id.textView1);
                  welcome.setText("Hello " + user.getBirthday() + "!");
                  Log.d("user_birthday","user_birthday:"+user.getBirthday());//user.getBirthday()  this value always is null?
                }
              }
            });
          }
        }
      });
  </code></pre>