[Modify] Polish it

websocket-sharp/Net/WebHeaderCollection.cs

@@ -831,9 +831,12 @@ namespace WebSocketSharp.Net
 
     private static string checkValue (string value)
     {
-      if (value == null || value.Length == 0)
+      if (value == null)
         return String.Empty;
 
+      if (value.Length == 0)
+        return value;
+
       value = value.Trim ();
 
       if (value.Length > 65535) {