title: Basic auth with Alamofire does not work
body: <p>I have been trying to use Alamofire's basic authentication support:</p>
  
  <pre><code>self.sessionManager.request(request)
      .authenticate(user: user, 
                    password: passwordHashAsHex)
  </code></pre>
  
  <p>However, I get a 403 from my webserver.  </p>
  
  <p>If I add the auth header my self:</p>
  
  <pre><code>let authString = "\(user):\(passwordHashAsHex)"
  
  guard let auth = authString.data(using: String.Encoding.utf8) else {
          throw ...
  }
  
  request.setValue("Basic \(auth.base64EncodedString())",
      forHTTPHeaderField: "Authorization"
  )
  </code></pre>
  
  <p>It works just fine.</p>
  
  <p>Checking <code>response.request!.allHTTPHeaderFields</code> in Alamofire's completion/response handler, I note that there is no entry <code>Authorization</code>. Why not?</p>