user's email must be unique, not case sensitive
update test to validete email umiquness
add index to users email for db. check email uniqueness
migrate db with add_index_to_users_email
downcase users email before save.
add bcrypt gem
add test to check user have password_digest
migrate db to add password_digest to users
add test for User having 'password' and 'password_confirmation'
add tests about 'password is not present' and 'password confirmation mismatch'
add test about 'password_confirmation is nil'
add tests for password authentication
update user model. add password, password_confirmation
update test. user requires password length at least 8