account identifiers need not be longer than 31 chars
Add shoulda-matchers dependency
Ensure the account is provided when saving a line
don't waste time validating the account

The database will do that for us
limit the length of transfer code
don't need to test what happens when code is nil
limit scope to 23 characters
ensure balance and amount columns are not null
range_type and function should not be nil
don't need shoulda-matchers after all