[Modify] Polish it

websocket-sharp/Net/HttpUtility.cs

@@ -764,12 +764,6 @@ namespace WebSocketSharp.Net
       if (response == null || response.Length == 0)
         return null;
 
-      if (credentialsFinder == null)
-        return null;
-
-      if (!(scheme == AuthenticationSchemes.Basic || scheme == AuthenticationSchemes.Digest))
-        return null;
-
       if (scheme == AuthenticationSchemes.Digest) {
         if (realm == null || realm.Length == 0)
           return null;
@@ -777,6 +771,13 @@ namespace WebSocketSharp.Net
         if (method == null || method.Length == 0)
           return null;
       }
+      else {
+        if (scheme != AuthenticationSchemes.Basic)
+          return null;
+      }
+
+      if (credentialsFinder == null)
+        return null;
 
       if (!response.StartsWith (scheme.ToString (), StringComparison.OrdinalIgnoreCase))
         return null;