`validates_confirmation_of` does not override writer methods.
`has_secure_password` is not invalid when assigning empty Strings.

Closes #9535.

With 692b3b6 the `password=` setter does no longer set blank passwords.
This triggered validation errors when assigning empty Strings to `password`
and `password_confirmation`.

This patch only sets the confirmation if it is not `blank?`.
Merge pull request #9545 from senny/9535_secure_password_blank

`has_secure_password` is not invalid when assigning empty Strings