HTTP API V2 download range request: strange behaviour

I don't know how to get raw requests, because they are generated by the following obj-c code.

+ (CloudHTTPResponse*) sendDownloadRequestWithToken:(NSString*)token path:(NSString*)path offset:(UInt64)offset andLength:(UInt64)length {
    NSString *url = @"https://content.dropboxapi.com/2/files/download";
    NSString *json = [NSString stringWithFormat:@"{\"path\": \"%@\"}", path];
    NSURL *urlWithArgs = [NSURL URLWithString:
                          [NSString stringWithFormat:@"%@?arg=%@", url, [json stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]]];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:urlWithArgs];
    [request setHTTPMethod:@"GET"];
    [request setValue:[NSString stringWithFormat:@"Bearer %@", token] forHTTPHeaderField:@"Authorization"];
    [request setValue:[NSString stringWithFormat:@"bytes=%llu-%llu", offset, offset+length-1] forHTTPHeaderField:@"Range"];
    return [self sendRequestSynchronously:request];
}

+ (CloudHTTPResponse*) sendRequestSynchronously:(NSURLRequest*)request {
    NSLog(@"%@", [request URL]);
    NSLog(@"%@", [request HTTPMethod]);
    NSLog(@"%@", [request allHTTPHeaderFields]);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    NSURLSession *session = [NSURLSession sharedSession];
    __block CloudHTTPResponse *httpResponse = nil;
    NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        httpResponse = [[CloudHTTPResponse alloc] initWithHeaderFields:[(NSHTTPURLResponse *)response allHeaderFields]
statusCode:(int)[(NSHTTPURLResponse *)response statusCode]
data:data
andError:error];
        dispatch_semaphore_signal(semaphore);
    }];
    [task resume];
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    NSLog(@"%i", httpResponse.statusCode);
    NSLog(@"%@", httpResponse.headerFields);
    return httpResponse;
}

As you can see, headers are printed right before sending the request and after receiving the response.