我测试发现POST请求也缓存了

```
NSURL *url = [NSURL URLWithString:@"http://ac-g3rossf7.clouddn.com/xc8hxXBbXexA8LpZEHbPQVB.jpg"];
// NSURLRequest *request = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:15.0f];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:15.0f];
[request setHTTPMethod:@"POST"];
NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {

NSDictionary *dic = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableContainers error:nil];
NSLog(@"%@",dic);
NSLog(@"%@",[NSThread currentThread]);
}];
[task resume];
```

请求后沙盒中同样缓存了图片。