User-Agent is missing from `NSURLRequest` in `stubRequestsPassingTest:...` when using `NSURLSessionConfiguraiton`?

I am setting up the userAgent string using `NSURLSessionConfiguraiton`:

```
NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
config.HTTPAdditionalHeaders = @{ @"User-Agent": @"Test User Agent" };
NSURLSession *session = [NSURLSession sessionWithConfiguration:config];
```

The User-Agent is being sent over the network - I can see it using Charles Proxy. However, it is not coming though in the stub checker:

```
[OHHTTPStubs stubRequestsPassingTest:^BOOL(NSURLRequest *request) {
NSDictionary* headers = request.allHTTPHeaders;
// `headers` does not contain the user agent entry
return YES;
} withStubResponse:^OHHTTPStubsResponse*(NSURLRequest *request) {
return nil;
}];
```