facho/fe/form(Invoice): se separa prefix y consecutivo.

* facho/fe/form(Invoice.set_ident_prefix): nuevo metodo para
asignacion manual del prefijo, por defecto se obtiene de
**ident**.

FossilOrigin-Name: 998d3315e15c83626d55bb8f3f2c2d16bf4c1c4a8ea172350254acfeb4ce6e4e

tests/test_form.py

@@ -174,3 +174,13 @@ def test_invoice_legalmonetary_with_taxes():
     assert inv.invoice_legal_monetary_total.charge_total_amount == form.Amount(0.0)
     assert inv.invoice_legal_monetary_total.payable_amount == form.Amount(100.0)
 
+def test_invoice_ident_prefix_automatic():
+    inv = form.NationalSalesInvoice()
+    inv.set_ident('SETP1234567')
+    assert inv.invoice_ident_prefix == 'SETP'
+
+def test_invoice_ident_prefix_manual():
+    inv = form.NationalSalesInvoice()
+    inv.set_ident('SETP1234567')
+    inv.set_ident_prefix('SETA')
+    assert inv.invoice_ident_prefix == 'SETA'