Fix WebHeaderCollection copying

Author: Meyn

DownloadAssistant/Options/WebRequestOptions.cs

@@ -49,7 +49,7 @@ protected WebRequestOptions(WebRequestOptions<TCompleated> options) : base(optio
             Timeout = options.Timeout;
             Headers = new();
             foreach (string key in options.Headers.AllKeys)
-                Headers.Add(key, Headers[key]);
+                Headers.Add(key, options.Headers[key]);
             UserAgent = options.UserAgent;
         }
     }

DownloadAssistant/Requests/WebRequest.cs

@@ -35,8 +35,12 @@ namespace DownloadAssistant.Requests
         protected HttpRequestMessage GetPresetRequestMessage(HttpRequestMessage? httpRequest = null)
         {
             httpRequest ??= new HttpRequestMessage();
-            foreach (string key in Options.Headers?.AllKeys ?? Array.Empty<string>())
-                httpRequest.Headers.Add(key, Options.Headers?[key]);
+            foreach (string key in Options.Headers.AllKeys ?? Array.Empty<string>())
+            {
+                string? headerValue = Options.Headers[key];
+                if (!string.IsNullOrEmpty(headerValue))
+                    httpRequest.Headers.TryAddWithoutValidation(key, headerValue);
+            }
 
             if (!string.IsNullOrWhiteSpace(Options.UserAgent))
                 httpRequest.Headers.Add("User-Agent", Options.UserAgent);