The proper way to specify Auto Sequence Transaction Entities

is to set a <AutoDocNumber /> node. Moreover, the <DocNumber>
element should not be present for this to work as expected. This PR is
just for Invoice. More here:
https://developer.intuit.com/docs/0025_quickbooksapi/0058_faq/qbo_v2_to_v3_migration_guide#Auto_Sequence_Transaction_Entities