Few fixes to the popup collection feature

I've made few fixes to the popup collection feature:

1. Converting the popup message to unicode instead of str (otherwise it will crash on unicode text).

2. Making sure the result of `Ghost._prompt_expected` is always a string, but if it's a boolean it raises an error (and it can be a boolean when using default_popup_response).

3. I removed the clearing of `Ghost._confirm_expected`/`Ghost._prompt_expected` in `GhostWebPage.javascriptPrompt` and `GhostWebPage.javascriptConfirm`, because it gets cleared anyway in the context handlers.