Update has_secure_password password setter

The instance variable `@password` should be updated even if the `unencrypted_password` parameter passed to the setter is blank. 

For example:

    user.password = 'mUc3m00RsqyRe'
    user.password_digest # => "$2a$10$4LEA7r4YmNHtvlAvHhsYAeZmk/xeUVtMTYqwIvYY76EW5GUqDiP4."
    user.password = nil
    # correct behavior would be
    user.password_digest # => nil
    # while without this file change you still get 
    user.password_digest # => "$2a$10$4LEA7r4YmNHtvlAvHhsYAeZmk/xeUVtMTYqwIvYY76EW5GUqDiP4."