Can't get HTTP headers from stubbed request

### New Issue Checklist

- [x] I have tried with the latest version of OHHTTPStubs
- [x] I have read the [README](https://github.com/AliSoftware/OHHTTPStubs/blob/master/README.md)
- [x] I have read the [Using the right Swift Version of `OHHTTPStubs` for your project](https://github.com/AliSoftware/OHHTTPStubs#using-the-right-swift-version-of-ohhttpstubs-for-your-project) section
- [x] I have searched in the [existing issues](https://github.com/AliSoftware/OHHTTPStubs/issues?utf8=✓&q=is%3Aissue)
- [x] I have read [the OHHTTPStubs wiki](https://github.com/AliSoftware/OHHTTPStubs/wiki) to see if there wasn't a detailed page talking about my issue

### Environment

- version of OHHTTPStubs: 6.1.0
- integration method you are using:
* [x] Cocoapods
* [ ] Carthage
* [ ] submodule
* [ ] other
- version of the tool you use: 1.4.0

### Issue Description

I always get request.allHTTPHeaderFields == nil. At least there should be an auto-generated user-agent header.

Here is my stub code with redirection to the same url with substituted headers to set my custom cookie. request.allHTTPHeaderFields == nil in both cases: before and after redirection.

```
[OHHTTPStubs stubRequestsPassingTest:^BOOL(NSURLRequest * _Nonnull request) {
return YES;
} withStubResponse:^OHHTTPStubsResponse * _Nonnull(NSURLRequest * _Nonnull request) {
//allHTTPHeaderFields is always nil
NSDictionary *allHTTPHeaderFields = request.allHTTPHeaderFields;
return [OHHTTPStubsResponse responseWithData:[NSData data]
statusCode:302
headers:@{@"Location" : downloadURL,
@"Set-Cookie" : @"cookie_name=value"
}];
}];
```

Network request code:

```
AFHTTPSessionManager *sessionManager = [AFHTTPSessionManager manager];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
//.....
NSURLSessionDownloadTask *downloadTask = [manager downloadTaskWithRequest:request progress:nil destination:nil
completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {
//....
}];
[downloadTask resume];
```