Return Decimal for default value of sign fields instead of integer

(transplanted from 2167000098d03b7e64de9a543d721240f661eaa6)

tax.py

@@ -367,16 +367,16 @@ class TaxTemplate(OSV):
         return False
 
     def default_invoice_base_sign(self, cursor, user, context=None):
-        return 1
+        return Decimal('1')
 
     def default_invoice_tax_sign(self, cursor, user, context=None):
-        return 1
+        return Decimal('1')
 
     def default_credit_note_base_sign(self, cursor, user, context=None):
-        return 1
+        return Decimal('1')
 
     def default_credit_note_tax_sign(self, cursor, user, context=None):
-        return 1
+        return Decimal('1')
 
     def get_active(self, cursor, user, ids, name, arg, context=None):
         return dict.fromkeys(ids, True)
@@ -598,16 +598,16 @@ class Tax(OSV):
         return False
 
     def default_invoice_base_sign(self, cursor, user, context=None):
-        return 1
+        return Decimal('1')
 
     def default_invoice_tax_sign(self, cursor, user, context=None):
-        return 1
+        return Decimal('1')
 
     def default_credit_note_base_sign(self, cursor, user, context=None):
-        return 1
+        return Decimal('1')
 
     def default_credit_note_tax_sign(self, cursor, user, context=None):
-        return 1
+        return Decimal('1')
 
     def default_company(self, cursor, user, context=None):
         company_obj = self.pool.get('company.company')