Preventing NSURLSession default HTTP headers

I'm trying to craft a very specific HTTP request to a server (ie. defining the exact set of HTTP headers), but NSURLSession keeps "helpfully" inserting a bunch of HTTP headers like Accept, Accept-Language and Accept-Encoding.

Consider the following playground (Swift 2.x) which sends a request to a service that just echos the HTTP headers that were sent:

import Foundation
import XCPlayground

XCPlaygroundPage.currentPage.needsIndefiniteExecution = true

let url = NSURL(string: "http://httpbin.org/headers")!
let request = NSMutableURLRequest(URL: url, cachePolicy: .ReloadIgnoringLocalCacheData, timeoutInterval: 30000)
let configuration = NSURLSessionConfiguration.ephemeralSessionConfiguration()
let session = NSURLSession(configuration: configuration)

let task = session.dataTaskWithRequest(request) { (data: NSData?, response: NSURLResponse?, error: NSError?) in
    print(NSString(data: data!, encoding: NSUTF8StringEncoding))
    XCPlaygroundPage.currentPage.finishExecution()
}
task.resume()

You can see that there are three Accept headers being sent. How can I prevent that?

I've tried setting the header using request.setValue(nil, forHTTPHeaderField: "Accept-Language") but that gets ignored. Tried setting it to "", but no good. I've also tried manipulating the HTTPAdditionalHeaders property on NSURLSessionConfiguration, but no love.

How do I get NSURLSession to not be quite so helpful?

I doubt what you're asking for is possible. NSURLSession (and NSURLConnection) automatically provide a number of headers, and that's one of them.

There's also no valid reason to remove them. All three of those headers have been part of the spec since the original HTTP/0.9 spec (https://www.w3.org/Protocols/HTTP/HTRQ_Headers.html). There's absolutely no excuse for any server not either handling those correctly or ignoring them outright.

With that said, if you provide the wrong value for those fields (and the default may be wrong), the server may refuse to give you results. To solve that problem, first figure out what type of data the server is actually going to provide, and specify that value in the Accept header instead of the default.

For example, you might set Accept to "application/json" or one of the other variants (What is the correct JSON content type?) if you're expecting JSON data.

That said, if you really must avoid having those headers sent, you can always open a socket, construct the request manually, and send it. Assuming the server doesn't require chunked encoding, it is pretty easy to do. (Chunked encoding, however, is a horror of Herculean proportions, so if your server sends that back, you'll pretty much have to resort to adding libcurl into your project and using that to make requests instead.)