The short answer: Use the version of `request` that takes a `URLRequestConvertible`, and specify the header fields in that.
The long answer: Custom headers on a per-request basis are a bit of a red herring, from an API design perspective. In reality, most use cases boil down to one of three, which are taken care of in other ways:
- Default headers, such as `User-Agent`, are specified in `NSURLSessionConfiguration -HTTPAdditionalHeaders`.
- `Authorization` headers, which are handled by `NSURLCredential` and `NSURLAuthenticationChallenge`.
- `Content-Type` headers, which are handled by `ParameterEncoding`.
