title: Set content-type when performing GET using Alamofire 4
body: <p>All I want to do is make a simple GET request using Alamofire 4 but set the content-type to application/json</p>
  
  <p>I cannot see a way of doing it on the request. How do I do this.</p>
  
  <p>I thought this would work but it doesn't.</p>
  
  <pre><code>Alamofire.request("https://dummy.com/dummy",
                             method: .get,
                             parameters: nil,
                             encoding: JSONEncoding.default,
                             headers: nil).responseJSON {
              response in
              print(response.request)  // original URL request
              print(response.response) // HTTP URL response
              print(response.data)     // server data
              print(response.result)   // result of response 
  }
  </code></pre>
  
  <p>I need to do this so that the service I am calling will return the data as JSON rather than XML. Any ideas?</p>