获取下载文件的大小，从而设置下载进度条

我们可以从http返回的response中拿到一些连接的信息。

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
if(httpResponse && [httpResponse respondsToSelector:@selector(allHeaderFields)]){
NSDictionary *httpResponseHeaderFields = [httpResponse allHeaderFields];

//把获取的文件的大小，存储在Content-Length里面。

total_ = [[httpResponseHeaderFields objectForKey:@"Content-Length"] longLongValue];

//NSLog(@"%lld", total_);
}
}