Implementing a way to get some useful debug info about why a form was bl...

...ackholed
I know you didn't change this, but should the hashed `$fields` variable be renamed here to (eg) `$fieldsHash` or simply `$hash`?

Using the latter, you'd be able to simply do `$debugData = compact('fields', 'unlocked', 'hash');` or even omit the `$debugData` variable and use `CakeSession::write('_Blackhole', compact('fields', 'unlocked', 'hash'));` and keep the "debug" code out of the regular code path.