Fix login - email can be pre-filled

[bigfox]

Hello, I have prepared PR with a fix for login.

I have a problem with one of my accounts. When this account tries to log in without a saved session, it's OK. But when try to log in with the saved session, the script ends with the error same as #93.

If I delete the session, everything works OK, until the next run. It's very annoying to delete a session every run.

After some debugging, I noticed that the login email may already be prefilled.

I added the check for prefilled email. If detected, then skip filling email and go fill password.