[Modify] Polish it

websocket-sharp/Net/WebHeaderCollection.cs

@@ -1809,11 +1809,11 @@ namespace WebSocketSharp.Net
     /// </exception>
     public void Set (HttpResponseHeader header, string value)
     {
+      value = checkValue (value);
+
       var key = header.ToString ();
       var name = getHeaderName (key);
 
-      value = checkValue (value);
-
       checkRestricted (name, HttpHeaderType.Response);
       checkAllowed (HttpHeaderType.Response);
       set (name, value, HttpHeaderType.Response);