Get content length from gzipped NSURLResponse

Question

I'm using NSURLSession's dataTaskWithRequest to perform an HTTP GET with Accept-Encoding: gzip. NSURLSession will automatically un-gzip the gzipped stream. How can I get the original response's content length (i.e. the length before un-gzipping)? Note that:

• NSURLResponse's expectedContentLength is the length after un-gzipping it.
• NSURLSessionTask's countOfBytesReceived property is the same value as expectedContentLength
• While the header Content-Length can be obtained from NSHTTPURLResponse, it's not available with HTTP 1.1's chunked transfer encoding.