to_hash method on BankAccount

Hi

Should line 24 of bank_account.rb have an each, e.g. currently it is

```
    self.class.attributes {|attribute| value = send(attribute); hash[attribute] = value if value}
```

Should it instead be:

```
    self.class.attributes.each {|attribute| value = send(attribute); hash[attribute] = value if value}
```

Cheers
