[Modify] Polish it

websocket-sharp/Net/WebHeaderCollection.cs

@@ -789,9 +789,12 @@ namespace WebSocketSharp.Net
 
     private static string checkName (string name)
     {
-      if (name == null || name.Length == 0)
+      if (name == null)
         throw new ArgumentNullException ("name");
 
+      if (name.Length == 0)
+        throw new ArgumentException ("An empty string.", "name");
+
       name = name.Trim ();
 
       if (!IsHeaderName (name)) {