使用第三方登录注册第二个用户时会失败，删除第一个用户后可以正常注册
错误日志：------ BEGIN LeanCloud REST Response ------
path: /1.1/users
cost: 510.848ms
response: Error Domain=com.LeanCloud.ErrorDomain Code=137 "A unique field was given a value that is already taken. class:User, index:shortId1, duplicate value:{ : null }" UserInfo={NSLocalizedFailureReason=A unique field was given a value that is already taken. class:User, index:shortId1, duplicate value:{ : null }, com.alamofire.serialization.response.error.data=<7b22636f 6465223a 3133372c 22657272 6f72223a 22412075 6e697175 65206669 656c6420 77617320 67697665 6e206120 76616c75 65207468 61742069 7320616c 72656164 79207461 6b656e2e 20636c61 73733a5f 55736572 2c20696e 6465783a 73686f72 7449645f 312c2064 75706c69 63617465 2076616c 75653a7b 203a206e 756c6c20 7d227d>, com.leancloud.restapi.response.error={
code = 137;
error = "A unique field was given a value that is already taken. class:User, index:shortId1, duplicate value:{ : null }";
}}
