Hotfix/fix multi thread error

[boycehe]

when call -(NSString *)formattedDateWithFormat:(NSString *)format in multi thread the result may be error for example:

for (NSInteger index = 0; index < 1000; index++) {

if (index % 2 ==0) {
  [NSThread detachNewThreadWithBlock:^{
   NSLog(@"test1:%@",[[NSDate date] formattedDateWithFormat:@"yyyy-MM-dd"]);
  }];
}else{
  [NSThread detachNewThreadWithBlock:^{
     NSLog(@"test2:%@",[[NSDate date] formattedDateWithFormat:@"yyyy-MM-dd HH:mm:ss.SSS"]);
  }];
}

}

the results is:

2018-04-13 14:07:14.355993+0800 TestDemo_Example[21257:35085629] test1:2018-04-13
2018-04-13 14:07:14.422793+0800 TestDemo_Example[21257:35085632] test2:2018-04-13
2018-04-13 14:07:14.422932+0800 TestDemo_Example[21257:35085631] test1:2018-04-13
2018-04-13 14:07:14.423280+0800 TestDemo_Example[21257:35085633] test1:2018-04-13 14:07:14.354
2018-04-13 14:07:14.423508+0800 TestDemo_Example[21257:35085634] test2:2018-04-13 14:07:14.354
2018-04-13 14:07:14.424189+0800 TestDemo_Example[21257:35085635] test1:2018-04-13 14:07:14.354
2018-04-13 14:07:14.424334+0800 TestDemo_Example[21257:35085637] test1:2018-04-13 14:07:14.354
2018-04-13 14:07:14.424579+0800 TestDemo_Example[21257:35085638] test2:2018-04-13