Extracted default HTTP header creation into a public class method

In the current implementation, I can't access logic for building those headers without creating the Alamofire SessionManager singleton like so.

```
Alamofire.Manager.sharedInstance.session.configuration.HTTPAdditionalHeaders
```

It seems that for those of us creating our own `SessionManager` subclasses, we should be able to access that header creation logic without having to instantiate the singleton object. This pull request extracts the header logic into a separate class method to allow subclassed `SessionManager` objects access.