         URLString:String,
         params:[String:AnyObject],
         data:[String:AnyObject],
-        json:[String:AnyObject]?,
+        json:AnyObject?,
         headers:CaseInsensitiveDictionary<String,String>,
         files:[String:HTTPFile],
         timeout:Double?,

        URLString:String,
        params:[String:AnyObject] = [:],
        data:[String:AnyObject] = [:],
        json:[String:AnyObject]? = nil,
        headers:[String:String] = [:],
        files:[String:HTTPFile] = [:],
        auth:(String,String)? = nil,

        URLString:String,
        params:[String:AnyObject] = [:],
        data:[String:AnyObject] = [:],
        json:AnyObject? = nil,
        headers:[String:String] = [:],
        files:[String:HTTPFile] = [:],
        auth:(String,String)? = nil,
