BCryptMigrationFromSHA1: Make sure we save the password after encrypting it to Bcrypt

The `BCryptMigrationFromSHA1` password strategy wasn’t saving the generated Bcypt password to database, so SHA-1 users were never migrated to Bcrypt.

In this pull request, we call `save` on the `User` model in `authenticated_with_sha1?` right after assigning the Bcrypt-encrypted password to `encrypted_password`.

See also issue #236 where I described the issue.