AFN中，去除NSNULL值

在网络请求中，返回了NSNULL，可以使用下面的代码，在设置baseUrl的时候，进行剔除。

- (id)initWithBaseURL:(NSURL *)url {
    self = [super initWithBaseURL:url];
    if (!self) {
        return nil;
    }
    
    self.requestSerializer = [AFJSONRequestSerializer serializer];
    [self.requestSerializer setValue:@"application/json" forHTTPHeaderField:@"Accept"];
    [self.requestSerializer setValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
    
    AFJSONResponseSerializer *JSONSerializer = [AFJSONResponseSerializer serializer];
    JSONSerializer.removesKeysWithNullValues = YES;
    self.responseSerializer = JSONSerializer;

    return self;
}