If Gson mapper is chosen, the oauth token response is empty about spring-rest-service-oauth HOT 2 OPEN

2016-03-25 00:37:08.102 DEBUG 71364 --- [           main] o.s.w.s.m.m.a.HttpEntityMethodProcessor  : Written [e51ff52a-853a-46f5-ad72-9500cb2e15d3] as "application/json;charset=UTF-8" using [org.springframework.http.converter.json.GsonHttpMessageConverter@20256a0b]
2016-03-25 00:37:08.102 DEBUG 71364 --- [           main] o.s.t.web.servlet.TestDispatcherServlet  : Null ModelAndView returned to DispatcherServlet with name '': assuming HandlerAdapter completed request handling
2016-03-25 00:37:08.102 DEBUG 71364 --- [           main] o.s.t.web.servlet.TestDispatcherServlet  : Successfully completed request
2016-03-25 00:37:08.103 DEBUG 71364 --- [           main] o.s.b.f.s.DefaultListableBeanFactory     : Returning cached instance of singleton bean 'delegatingApplicationListener'
2016-03-25 00:37:08.103 DEBUG 71364 --- [           main] o.s.s.w.a.ExceptionTranslationFilter     : Chain processed normally
2016-03-25 00:37:08.103 DEBUG 71364 --- [           main] s.s.w.c.SecurityContextPersistenceFilter : SecurityContextHolder now cleared, as request processing completed

MockHttpServletRequest:
      HTTP Method = POST
      Request URI = /oauth/token
       Parameters = {username=[craig], password=[spring], grant_type=[password], scope=[read write], client_id=[clientapp], client_secret=[123456]}
          Headers = {Authorization=[Basic Y2xpZW50YXBwOjEyMzQ1Ng==], Content-Type=[application/x-www-form-urlencoded]}

Handler:
             Type = org.springframework.security.oauth2.provider.endpoint.TokenEndpoint
           Method = public org.springframework.http.ResponseEntity<org.springframework.security.oauth2.common.OAuth2AccessToken> org.springframework.security.oauth2.provider.endpoint.TokenEndpoint.postAccessToken(java.security.Principal,java.util.Map<java.lang.String, java.lang.String>) throws org.springframework.web.HttpRequestMethodNotSupportedException

Async:
    Async started = false
     Async result = null

Resolved Exception:
             Type = null

ModelAndView:
        View name = null
             View = null
            Model = null

FlashMap:
       Attributes = null

MockHttpServletResponse:
           Status = 200
    Error message = null
          Headers = {X-Content-Type-Options=[nosniff], X-XSS-Protection=[1; mode=block], Cache-Control=[no-cache, no-store, max-age=0, must-revalidate, no-store], Pragma=[no-cache, no-cache], Expires=[0], X-Frame-Options=[DENY], Content-Type=[application/json;charset=UTF-8]}
     Content type = application/json;charset=UTF-8
             Body = {}
    Forwarded URL = null
   Redirected URL = null
          Cookies = []
2016-03-25 00:37:08.158 DEBUG 71364 --- [           main] c.jayway.jsonpath.internal.CompiledPath  : Evaluating path: $['access_token']