As far as I can tell there is no sane way to do line items as QBXML

wants them given a hash. For example QBXML wants
<InvoiceLineAdd>
  ....
</InvoiceLineAdd>
<InvoiceLineAdd>
  ....
</InvoiceLineAdd>

But all I can get using a Hash is a parent wrapper around that such as:
<LineItems>
  <InvoiceLineAdd>
    ....
  </InvoiceLineAdd>
  <InvoiceLineAdd>
    ....
  </InvoiceLineAdd>
  ..

This commit will allow the usage of a parent hash node called
"line_items:", which will not be outputed in the final XML.

See spec/lib/qbxml/hash_spec.rb